128. Max. 1883, October 6. 132. Min. 1884, August 10.

Magn. 9.6.
Magn. 12.7.

1884, September 15. Magn. 9.0. K. Max. 1884, October 15. Magn. 7.6.

K.

1326. Max. 1883, February 6. Magn. 6.3. 1884, March 7. Magn. 6.8. Min. 1883, August 23. Magn. 9.9. 1884, August 11. Magn. 9.7. K.

142. Max. 1883, December 25 (approximate). Magn. 6.3. Sr.

Table II. enables the number of any star in Table I. to be found from its usual designation. The constellations are arranged alphabetically in the first column, and the numbers are placed under the respective headings R, S, T, U, V, and W. Thus R, S, and T Aquarii are respectively Nos. 142, 138, and 131 in Table I. In the column headed "Misc." are given other designations; thus 7 Aquila is No. 118, and ʼn Argus is No. 54. The number is preceded by a dash when no letter has been definitely assigned to the corresponding star; thus, -,52c in the second line indicates that a variable star in Antlia, No. 52c in Table I., has not been definitely designated by a letter.

η

η

The list of suspected variables, given in Table II. of the statement made in 1884, is not here repeated, since from the nature of the case such a list can only be provisional. It should be remarked that all the stars which were inserted on the authority of Dr. Peters are cousidered by him to be certainly variable. Dr. Dunér also furnishes the place of a variable star which he observed on 23 days in 1884; it was not given in either of the tables published last year. The place, reduced to 1875, is as follows: R. A. 14h. 24m. 41s., 8 +39° 25'.2. The period is about 405 days. Four variable stars were detected by means of the observations made for the Cordoba Zone-Catalogue; see that work, p. xiv., and Dr. Gould's letter in the Astronomische Nachrichten, CXI. 63. The places for 1875 are as follows: R. A. 15h. 45m. 22s., Dec. -35° 55'.3; R. A. 22h. 10m. 53s., Dec. -30° 13'.6; R. A. 22h. 27m. 4s., Dec. 67° 55'.9; R. A. 23h. 49m. 58s., Dec. -50° 28'.9.

Table III. indicates the progress of observation of suspected variables given in Table II. of the statement made in 1884. The stars are designated in the first column by Mr. Chandler's provisional numbers, as in the previous statement. The second column gives the number of observations made by each observer, as in the last column of Table I. The third column indicates the result of these observations.

It is hoped that observers of variable stars will continue to furnish accounts of their work during each year as soon as possible after its close. It is desirable that these accounts should be received at the Harvard College Observatory as early as February 1 of the following year.

INVESTIGATIONS ON LIGHT AND HEAT, MADE AND PUBLISHED WHOLLY OR IN PART WITH APPROPRIATION FROM THE RUMFORD FUND.

XVIII.

A PHOTOGRAPHIC STUDY OF THE NEBULA OF

ORION.

BY EDWARD C. PICKERING.

Presented March 11, 1885.

No portion of the heavens has been more carefully studied than that containing the Nebula of Orion. The monographs by Prof. G. P. Bond (Annals Harvard College Observatory, V.) and by Prof. E. S. Holden (Washington Astronomical Observations for 1878, Appendix I.) show the vast amount of material collected by eye observations. For a photographic study of the same region the following specimens are in the photographic collection of the Harvard College Observatory:

A. Artotype enlargement of the first photograph of the nebula taken by Dr. Henry Draper, September 30, 1880. Exposure, 51 minutes. B. Artotype enlargement of a photograph taken by Dr. Henry Draper, March 11, 1881. Exposure, 106 minutes.

C. The original negative taken by Dr. Henry Draper on March 14, 1882. Exposure, 136 minutes. This negative, except for a slight photographic blemish, is nearly identical with that from which D. was taken.

D. An enlarged glass positive of the second photograph taken by Dr. Henry Draper, March 14, 1882. Exposure, 137 minutes. This positive is a duplicate of that employed in making the paper prints, E. The two positives were taken, and any objects resembling stars, but not found on both, were assumed to be defects, and were painted out of the other positive by Dr. Draper.

E. Several artotype enlargements of the second photograph, taken March 14, 1882, by Dr. Henry Draper.

F. Carbon print of photograph taken by Mr. Common with his 3-foot reflector, January 30, 1883. Exposure, 39 minutes. Enlargement about 7 times.

G. Glass positive, a direct copy of the negative taken by Mr. Common with his 3-foot reflector, February 26, 1883. Exposure, 60 minutes.

Admirable material is thus furnished for a comparison of the results of photographic and eye observations of this region. The photographs of the stars which are common to F and to the catalogue of Professor Bond (Annals, V. 270) were first compared by a method closely resembling that adopted by Argelander for the study of variable stars. Table I. gives the stars which were selected for standards, with which the others are to be compared. Each star in the photograph was then compared with two of these, one a little brighter, the other a little fainter. The differences were estimated in grades. The sum of the two differences gave a measure of the interval between the two comparison stars. It frequently happened that no difference in brightness was perceptible between the star to be measured and one of the comparison stars. The number of measures of star intervals between the comparison stars is therefore less than the number of stars compared. In Table I. the successive columns give for each comparison star a designation, and the number and magnitude in the Bond catalogue. The next column gives the photographic magnitude, found by a process which will be detailed below. This is followed by the number of comparisons between each star and that following it, and the mean value of this difference in grades. The last column gives the assumed brightness in grades, and equals the number of grades by which each star is fainter than the first on the list.

The light of each star measured was next reduced to grades by the assumed light in grades of the comparison stars. Two values were found,- one derived from the brighter, the other from the fainter comparison star. In 50 cases the results agreed exactly, in 11 cases