60

doublet the front lens is approximated much closer to the object than it possibly could be if employed alone, and hence it admits a wider angle; the reduction of magnifying power, at the same time, diminishes spherical aberration, which is still further reduced by the peculiar relations of the curvatures. The doublet thus becomes a very superior instrument, and, when well made and carefully used, surpasses all but the most improved forms of the compound instrument. Doublets of gems are far superior to those of glass. Many years since the writer made several garnet doublets of to of an inch focal length, which in performance on test objects quite equalled the best French achromatics of the day. Triplets are superior to the doublets; and for a simple microscope the achromatic triplets now furnished by the French and German opticians, as objectives for the compound microscope, will be found very effective. Essentially, the English and American achromatic objectives are triplets, but the peculiar mounting of these instruments prevents their use as simple microscopes. So great has been the improvement, that the best modern objectives will transmit angular pencils of 170° to 178°. We may notice here the so called "Coddington lens," or grooved sphere. It is an invention of Sir David Brewster, and when properly made is almost free from spherical aberration, and the chromatic aberration is almost insensible. "It consists of a spherical lens, or sphere with a deep concave groove cut round it, so as to cut off the marginal pencils, and thus give a wider field and more perfect image." The lenses usually sold under this name are simply cylinders of glass having spherical ends, and of course have none of the advantages of large field and freedom from spherical aberration proposed by Dr. Brewster. When the curvatures of this cylindrical lens are unequal, and such that, the most convex being turned toward the eye, an object placed on the other convex surface is in the proper focus of the lens, it is called a "Stanhope lens;" its use is limited to such objects as can be directly applied to the surface. When of considerable power it may be advantageously employed in searching for diatomacea; the drop of water supposed to contain them may be examined by applying it to the least convex surface. All the simple microscopes, and especially the higher powers, require some kind of a stand or carrier. The lower powers and single lenses are usually attached to the end of a jointed rod, which can be moved up and down a stem inserted into a solid base. The most convenient mounting for a 1-inch or 4-inch lens for the purpose of preliminary examinations, or botanical dissections, is that of Messrs. Powell and Leland of London, and is employed by them as the mounting of the small condenser for their compound instrument. The movements are complete, and one can place the lens, whatever may be the position of the object, in such a relation to it as will insure the best view. For the higher powers, to of an inch, a steady

well made stand will be required, and some means of adjusting the focus delicately, either by rack-work or screw. Various forms have been devised by different opticians; perhaps, upon the whole, that known as the "Raspail' is most simple, and at the same time of great excellence. It consists of a brass pillar, up and down which a large circular stage is moved by rack-work; a large mirror, one side plane, the other concave, swings freely below, and serves to direct the light upon the object; at the top of the brass pillar is placed the lens holder, movable forward by means of a screw, and laterally by swinging round a pin inserted in the top of the pillar; into the opening of the stage is fitted a glass plate, or it may be made to hold dissecting troughs with glass bottoms. It is often convenient, or absolutely necessary, for the examination and dissection of opaque objects, to have the lens inserted in a silver cup, or Lieberkuhn, which, receiving the light from the mirror below, reflects it back, condensed, upon the object. These Lieberkuhns are usually made of silver. The very simple microscope employed by Ellis in his researches on corallines, and in which all the adjustments were effected by sliding by the hand, was fitted with these silver cups. Although the Lieberkuhn is very commonly applied to the low power achromatic objectives, it is now seldom to be obtained with any form of simple instrument, unless by special order; it will be found of the greatest service in minute dissection. In using lenses of moderate foci, to inch, the most extended distinct field is obtained when the convex side is presented to the object; but the sharpest vision of a minute point or small object, in the centre of the field, is when the flat side is presented to the object. In estimating the magnifying power of single lenses, an arbitrary standard of the nearest distance at which the healthy unassisted eye can view distinctly minute objects is assumed; this distance has been placed at from 5 to 10 inches. The latter is the standard adopted by most opticians and authors; Sir David Brewster alone adopts 5 inches. The magnifying power is usu ally expressed lineally, or as so many diameters." Thus, when the magnifying power is stated to be 40, it is meant that the diameter is increased 40 times, but of course the area would be increased 1,600 times. The following table exhibits the linear and superficial magnifying power, adopting the standard of 10 inches:

66

Superficial magnifying

power.

5

25

6.6

43.5

10

100

13.8

176.8

20

400

40

1,600

80

6,400

100

10,000

200

40,000

As it is difficult to measure exactly the solar focal length of small lenses, a sufficient approximation may be had by the method proposed by Mr. Ross, which answers admirably for doublets

and triplets. It consists in "viewing the image of some distant object formed by the lens in question, through another lens of one inch solar focal length, keeping both eyes open, and comparing the image presented through the two lenses with that of the naked eye. The proportion between the two images so seen will be the focal length required. The panes of glass in a window, or courses of bricks in a wall, are convenient objects for this purpose." The comparative focal lengths of two lenses, or sets of lenses, may be determined by holding them at the same distance from the eye and estimating the sizes of the images formed by each of the same object; thus, if one lens forms the image half the size of the other, lineal measure, its focal length is half that of the other. The same method applies to eye pieces.-We pass now to the compound microscope, and shall dwell only upon the construction and use of the instrument in its most approved form. For a history of the earlier forms, the reader may consult the elaborate works of Quekett and Harting, and the older works of Adams and Baker. Essentially it consists of two parts, the object glass and the eye piece. The former is now made by a combination, usually, of 3 sets of achromatic doublets, arranged to give the greatest freedom from spherical and chromatic aberration; the latter, of two plano-convex lenses, with the plane sides to the eye, the lens nearest the object, or "field lens," being almost exactly double the focal length of the eye lens, and the distance between them a little more than the focal length of the field lens; the ratio is varied somewhat by different makers. The eye piece thus formed is termed a "negative eye piece," or the "Huyghenean." The eye piece of Kellner is a decided improvement; it is termed "orthoscopic," and the eye lens is achromatic or nearly so; these eye pieces are supplied by the Messrs. Grunow of New Haven, Conn., with their best instruments; the field of view is large, free from distortion, and well defined throughout the whole extent. The orthoscopic eye piece supplied by Mr. Charles A. Spencer, and more recently as improved by R. B. Tolles, of Canastota, has both eye and field lens achromatic, and is exceedingly perfect; it is, however, more expensive than the Kellner eye piece. Mr. Tolles has also introduced very recently a solid, orthoscopic, negative eye piece, of remarkable clearness and definition throughout, especially fitted for micrometric use, the engraved scale being cemented in the body of the solid eye piece, and perfectly protected from all dust or interference with definition, so noticeable in the use of the eye-piece micrometer in the ordinary way. So far as we are aware, the English opticians have not yet adopted the improved eye pieces, but have devoted all their skill to the perfection of the objectives and the stands. The "Jurors' Report" of the exhibition of 1851 mentions a solid eye piece, a positive triple achromatic, very thick; but it appears not to have come into general use. Mr. Tolles has also

introduced what is termed an amplifier, being an achromatic concave of peculiar construction, which is introduced within the body of the microscope by means of an adapter. The corrections of the objective are not in the least disturbed by this arrangement, but the power is doubled. A low eye piece thus gives as much amplification as a higher one, but with the very great advantage of almost perfect flatness of field. The object glasses, or objectives" as they are now very commonly termed, derive their denominations, 1 inch, inch, inch, &c., from the fact that the combined sets of lenses give a magnifying power the same as a single lens of the same name. Thus, a object glass should give the same amplification as though a single lens of of an inch was used in its place. This term does not refer at all to the working distance, for, as is the case with doublets, the working distance with all powers higher than the inch is considerably less than that of the equivalent single lens; it will be apparent that for any given focus the working distance will, in general, be diminished by an increased angle of aperture; a of 90° will have in this respect a very great advantage over a of 140°. As regards the merits of the large angle objectives, there are various opinions. Dr. Carpenter is decided in his condemnation, considering that depth of penetration cannot be had at the same time with enlarged angle. The skill of the first opticians, Spencer, Tolles, and Grunow in America, Ross, Powell and Leland, and Smith, Beck, and Beck in England, have proved the contrary. Nor is Sir David Brewster's assertion true of large angle objectives, that they give a distorted view. The definition, clearness, and perfectness of vision with Powell and Leland's employed with aving an angle of 176°, when a low eye piece so as to give the same amplification as a with a higher, is greatly superior to that of the. The chief advantage of the small angle has been considered to be the sort of general view it would give of the whole of a minute object; the working distance being so great, that the minute elevations and depressions in the object itself, being but a very small fraction of the whole distance, would not perceptibly affect the focus. In many respects, this might be a desirable quality; but we are convinced that the microscopist would quite as often find it a source of error in his interpretation of what he might observe. We feel fully assured, that in minute and elaborate investigations the high angle objectives are the most trustworthy. Moreover, the skill of the opticians named has enabled them to increase the angle without diminishing so very much the working distance. The of Mr. Ross, of recent make, and we believe the of that of Powell and Leland, will work through glass .01 (1) of an inch in thickness.-In speaking of the objectives of the prominent makers, we do not desire to indicate any order of precedence; they are all excellent, and all have peculiarities of their own. We must, however,

12

be permitted to notice a little more fully the American artists. The oldest optician, and the one who has been most known in connection with the microscope in the United States, is Mr. Charles A. Spencer of Canastota, N. Y. The object glasses furnished by this gentleman, particularly the later ones, are of the highest order; they range from 3 inch to inch. By many of our most experienced microscopists they are considered superior to the best objectives of the London opticians; they are certainly equal to them. Mr. Spencer, in the earlier days of high angle objectives, no doubt surpassed in this respect all the English opticians; his rare skill and nice manipulation enabled him to perform wonderful feats in this direction, far in advance of any thing before accomplished. Mr. Robert B. Tolles, for some time connected with Spencer, but now by himself at Canastota, has devoted himself to the perfection of the achromatic objectives with enthusiastic zeal and unparalleled success. His recent object ives are quite equal in defining and penetrating power to the very best of the London opticians, both with central and oblique illumination, and greatly superior to them in the latter case, as regards chromatic aberration; this is true also of Mr. Spencer's objectives. Mr. Tolles's objectives range from 3 inch to Messrs. J. and W. Grunow of New Haven, Conn. (the former now of New York) have sent out some very fine objectives, ranging from 2 inch to inch. They have not attempted generally so high angles as Spencer and Tolles, but have devoted great attention to the mechanical arrangement and efficiency of their stands, and the accessory apparatus. In this department especially they are unrivalled, and they alone, of the American opticians, have adopted Mr. Wenham's superior mode of adjustment of the high power objectives for thickness of cover.Of English opticians, the name of Andrew Ross has always been placed foremost, being connected with the greatest improvement of the objective, without which the higher powers of large angle would be almost valueless; we allude to the adjustment for cover. First of all the opticians Mr. Ross made his objectives so perfectly corrected for spherical and chromatic aberration, that a new source of difficulty, apparently almost insurmountable, presented itself. He found that these aberrations, so nicely balanced, were disturbed by each varying thickness of the thin covering glass over the object. The happy expedient he devised to remedy this, was to alter the distance between the first set and the two posterior sets of achromatics composing the objective, by means of a delicate screw collar. This grand and capital improvement, for which Mr. Ross deserves the rank assigned him, has been adopted by all the American and English opticians, and more recently by the French and German. The English uniformly, if we except the amateur efforts of Mr. Wenham, make the front set movable; the American opticians move the two

posterior sets, the front being immovable; the latter method is better, the object being kept easier in view during the adjustment, and there being no danger of bringing the front lens in contact with the object. The objectives of Mr. Ross have always maintained a high character, and have been more expensive than those of Powell and Leland, or Smith, Beck, and Beck, though they probably do not surpass them. The business is now conducted by Mr. Thomas Ross, the son of A. Ross, who has lately died. Mr. Ross's objectives range from 3 inch to inch. Messrs. Powell and Leland, so far at least as the objectives and accessories are concerned, are quite equal if not superior to any living opticians. Both the low and high powers of this firm are of the finest character. While the general plan is the same, there are some peculiarities in their high power objectives worthy of notice. The front set is triple, and the front lens being of crown glass is less liable to injury from wiping, or accidental contact with the object, than the soft flint of other opticians; but as it is a very thin plano-convex, merely cemented to the concave, and not burnished in, it is liable to injury by parting the cement. Messrs. Smith, Beck, and Beck are more widely known in this country than the other firms. Their "students'" and "educational microscope" are the forms usually sold by the dealers. As opticians, they stand side by side with the others named; as mechanicians, in some respects, they appear to be in advance. They do not furnish objectives higher than of 125° angle. We have space only to mention the names of Nachet, Oberhauser, and Kellner, whose objectives are excellent, but inferior to the American or English; they are, however, much cheaper, and the mechanical work upon their instruments is very excellent. The name of Prof. Amici of Modena has long been associated with the microscope. The objectives made by him consist of 6 series, with angles varying from 26° to 160°; the higher powers have no adjustment for cover, but the front lens is made slightly concave, and a drop of water introduced between the cover and the objective, thus in a measure rendering the adjustment unnecessary. This contrivance is ingenious but awkward, and is a poor substitute for that suggested by Mr. Ross. Finally, we must not omit to name Mr. Lister, who first pointed out some peculiarities possessed by a combination of 3 achromatics with their plane sides toward the object, the crown and flint being cemented together, and which were the basis of subsequent improvements. The peculiar form adopted by Lister has long since been abandoned, the principles, however, being the same.-The stands furnished by the principal makers exhibit a great variety of patterns, and combine various excellences. The most desirable points, viz., freedom from tremor, ease of illumination, particularly oblique illumination, facility in the application of the accessory apparatus, and delicacy of adjustment, are nearly

equal in the first class stands of the principal opticians; but perhaps, upon the whole, the best are those of Messrs. Smith, Beck, and Beck. Their large stand mounted on two pillars is remarkably steady. The stage, thin but substantial, is large, and its movements are very smooth and delicate. The illuminating and accessory apparatus is carried by a sub-stage below, and most easily applied. The illuminating mirror is large, and so mounted as to give very oblique illumination. The rack-work of the quick adjustment is always of the smoothest and best character in all of the instruments made by this firm, and the slow adjustment very delicate. The greatest fault in this stand is the attachment of the slow movement at the lower part of the tube of the microscope itself. In this respect, the stands of Ross, Powell and Leland, Spencer, Tolles, and the Messrs. Grunow, are superior. With the high powers, the springing of the tube when the finger is applied, and the shaking when the adjustment for cover is attempted, are disagreeable. The larger students' microscope made by this firm is a very fine instrument; the stage movements and adjustments are very complete. The stands furnished by Ross are too heavy and cumbersome; they are, however, exceedingly steady, and finished with extreme care. Those supplied by Powell and Leland are much lighter, and have some peculiar advantages. The accessories supplied by this firm are more complete than those of any other; the stage of their new and largest instrument is very thin, and allows greater obliquity of illumination than that of Smith, Beck, and Beck. Their achromatic condenser is beautifully finished; it has an angle of 170°. The stand itself is not as steady as that of Smith, Beck, and Beck, but the fine adjustment is more conveniently placed. The stands furnished by Messrs. Grunow are of different patterns, but all excellent in beauty of finish, smoothness of adjustment, and steadiness, comparing most favorably with the English work. The stands furnished at Canastota by Spencer and Tolles have many excellences, and are very steady. Their more recent stands evince a most decided improvement in general finish and in the accessories, and we doubt not that the American stands will soon, as already do the American objectives, to say the very least, fully equal the best English work.-All good instruments should have a graduated drawtube within the main tube, and the latter should be not less than 1.4 inch in diameter. As a general rule, the American microscopes have had too small and short bodies, so that they have appeared, by the side of the English instruments, little and inferior. The draw-tube is absolutely necessary for micrometry, and is very convenient to receive the analyzing prism, erector, or Tolles's amplifier. Two adjustments for focus are also necessary, one quick by rackwork and pinion, the other very delicate by screw. The pinion heads should be large, to allow of most delicate movement, and Messrs.

Smith, Beck, and Beck graduate the head of the screw of the fine adjustment, so that the thickness of covering glass may be measured. A skilful observer may dispense with the rack movement, and produce the approximate adjustment by sliding the tube. This is the method adopted in most of the French instruments, and in the "educational" of Smith, Beck, and Beck. The stage movements should be smooth, and but slightly disturb the adjustment when in focus. The adjustment for wear in the English instruments is by spring, in the American by screws; the former involves more work for the maker, but is better. The lever stage, though performing finely when first from the hands of the maker, is much more liable to derangement than that in which the motions are produced by rack and screw. In the Smith, Beck, and Beck instruments the rack and screw are both so low that the latter passes entirely under the bottom of the stage, which is consequently much thinner than it can be made when the screw is introduced between the movable plates, as is the case with most of the American instruments; the milled heads themselves are thus dropped below the level of the stage, and this is deemed of great importance; this is also the case with Powell and Leland's large microscope.-The usual accessories accompanying the microscope are, an achromatic condenser, a bull's-eye condenser, small condenser, stage and eye-piece micrometers, polarizing apparatus, camera lucida, animalcule cage, stage forceps, glass parabola, erector, Lieberkuhns and dark wells, frog plate, &c. The name of achromatic condenser has been given to an illuminating apparatus consisting of an achromatic objective of large angle, furnished with a wheel of diaphragms and central stops; when the latter are used, oblique illumination is obtained. This condenser is exceedingly useful in the ordinary studies by the microscope. With the proper adjustment it affords a fine achromatic illumination, revealing the structure of the object with great beauty and clearness; but in unskilful hands it will be of little service. So far as oblique illumination is concerned, we prefer unilateral light, though some particulars of structure are best revealed by aid of the achromatic condenser. The condenser is placed below the main stage, and can be adjusted by independent rack-work, so as to give the best illumination. The condenser of 170° angle, lately introduced by Powell and Leland, is spoken of very highly by the English microscopists; the delicate markings of navi cula rhomboides, when viewed with their objective, are stated to be as well defined as those of pl. hippocampus viewed with a 1. The achromatic condenser of Smith, Beck and Beck has an angle of about 100°. When used with the central stops, the proper point for adjustment may be determined by removing the eye piece, after the objective has been brought into focus, looking down the tube, and moving the condenser by means of its own rack and pinion, until the black stop appears sharply defined in

the centre of the field, and a bright illuminated ring around it. If the stop is removed, the whole field will appear brightly illuminated, unless the objective be of much greater angle than the condenser. A condenser of greater angle than 100° cannot be used unless the object be placed upon very thin glass; all difficult test objects should be mounted in this manner on a slip of mahogany. The condenser itself, or the carrier, should have some means of adjustment to make its axis coincide with that of the objective; the front set of lenses is generally made to be removed, thus affording an illuminator of lower angle, to use with thicker glass and objectives of low power. In using the achromatic condenser by lamp light, the bull'seye condenser is employed to render the rays parallel, and the plane side of the mirror is used. -The bull's-eye condenser consists of a thick plano-convex lens, of short focus, mounted upon a stand so that it can be used for the illumination of opaque objects. Very excellent oblique illumination may be obtained by condensing the flame of a candle or lamp, placed at a distance of about two feet from the stage, upon the under surface of the slide, the light being placed in front, and the stage slightly turned up to receive it; or, the microscope being placed horizontal, the light may be off at one side, according to the obliquity desired. As the thickness of the stage will not ordinarily allow illumination at a much greater distance than 60° from the axis, a sub-stage, attached to the upper plate of the main stage, but carrying the slide entirely below it, is furnished by Messrs. Grunow, and by Powell and Leland, and may be readily attached to any microscope; this allows illumination of any obliquity. In using the bull's-eye condenser for this purpose with a lamp, the plane side must be presented toward the object; and if the flame be flat, its edge must be presented. The small condenser is used in the same way as the bull's-eye. Where very intense illumination is desired, the bull's-eye is placed near the source of illumination, with the plane side toward it, so as to render the rays nearly parallel, and then this beam of light is further condensed by the small condenser.-The stage micrometer is a slip of glass ruled into .01" and .001" of an inch; it is used in conjunction with the camera lucida, or to determine the value of the divisions of the eye-piece micrometer. The French scale is the millimètre divided into 100 or 200 parts. The eye-piece micrometer is a finely ruled glass scale, introduced by means of an opening between the field and eye lens of the eye piece, so as to be in the focus of the eye lens. The value of the scale is determined by placing the stage micrometer on the stage, and viewing the divisions with the given objective and eye piece, causing by means of the draw-tube a certain number of divisions of the one to correspond exactly with a certain number of the other; thus, if 10 divisions of the eye micrometer corresponded with one (.01") of the stage micrometer, then

the value of one division o. the eye-piece micrometer would be .001", with that particular object glass, eye piece, and length of draw-tube. The stage micrometer being replaced by any object, its dimensions may be readily ascertained by noticing how many divisions of the scale are subtended by it. The micrometers of this description are very convenient, and, when carefully used, accurate. The definition is slightly injured, however; this objection is obviated by Mr. Tolles's solid micrometer eye piece. If the observer has only a stage micrometer, the divisions may be projected on paper by means of the camera lucida; then, with the same objective and eye piece, the image of any object being projected on the paper, its dimensions are at once ascertained. Messrs. Powell and Leland furnish a cobweb micrometer susceptible of great accuracy; it is similar to that used for astronomical purposes. Messrs. Grunow have somewhat improved upon Powell and Leland, and their cobweb micrometer with orthoscopic eye piece is a very delicate instrument. They also furnish Fraunhofer's stage micrometer, which possesses the advantage of giving the absolute dimensions of the object, without reference to the power of the objective or eye piece. With careful use the eye-piece micrometer is as accurate as any of these, and much less expensive. The polarizing apparatus consists of two Nichols prisms of calc spar with revolving fittings, one (called the polarizer) designed to be placed below the object, the other (termed the analyzer) above, either directly over the eye piece, where it generally cuts off part of the field, or at the lower end of the draw-tube; here, if the prism be a good one and not too long, it will not much affect the definition, and will allow the whole field to be visible. Very much depends upon these prisms. Those supplied by the Messrs. Grunow are very large and fine, and the analyzer is placed by them directly above the objective; it is too large, however, to be placed so low down without injury to the definition. The analyzer of Smith, Beck, and Beck is much shorter than Grunow's, and may be used either over the eye piece or placed in the end of the draw-tube. There is much difference in these prisms; some scarcely injure the definition at all, and others are very poor. The polarizer is usually somewhat larger than the analyzer. The calc spar is very soft, and, if not protected by thin glass covers, as are those of Messrs. Grunow, liable to injury. A set of revolving selenites, to go below the object, between it and the polarizer, is supplied both by Grunow and by the English opticians. There is not much choice in the method of mounting and revolving these prisms; perhaps Grunow's is the most elaborate, but, when the revolving selenites are included, we think Smith, Beck, and Beck's arrangement the most complete. A "selenite stage" is often employed, simply placed under the object, on the stage of the microscope. Mr. Darker has contrived a stage of this kind, in which the