length, both are annihilated, and darkness is produced. These phenomena are admirably illustrated by Newton's rings. The rings are made by pressing a convex lens on a piece of plane glass. Newton's rings, viewed by the monochromatic sodium light, appear alternately light and dark. Darkness will only be produced with monochromatic light. When ordinary daylight is used, the rings appear coloured, because this light being composed of rays of different wave-lengths, only a portion of the light will be destroyed, and so colour is produced. The colours produced by diffraction, thin plates, and polarization are due to interference.

The above are the commonest ways in which colour is produced, but colour may be produced by alteration in wave-length, as in fluorescence, or by light which has been absorbed being given out again, as in phosphorescence.

It is therefore evident that in the absence of light colour is not produced, and that colours having a similar appearance to the normal-sighted may have a very different composition, as far as the physical structure of the colour is concerned. When I speak of the physical structure of a colour, I mean the composition of the light which is reflected from or transmitted by a coloured body.

If we take a small portion of the spectrum produced by a good spectroscope we shall obtain a band of apparently monochromatic light. If shutters be used to the spectroscope we shall not be able to say which is the red and which the violet side of the spectrum. By this means we obtain the purest possible colour. As has been previously explained, the band is really made up of numerous waves of light which differ from each other in wave-length and refrangibility. For comparison let us take twelve yellow bodies, choosing those which are as nearly as possible similar in every respect. Then we find that the same

of ways.

colour may be produced in an almost innumerable number Let us first deal with solar light. If this falls upon a body and the particles of that body absorb some of the blue rays it will appear yellow. It will therefore be seen that the composition of this yellow consists of red, orange, yellow, green, a few blue, and the violet rays of the spectrum. Gaslight is a good example of yellow light of the above composition. It is rare to find a yellow body which reflects only the yellow rays of the spectrum. Most yellow bodies reflect the orange rays and some of the red and green. We may even have a yellow which contains no yellow rays at all but is only a mixture of red and green

waves.

CHAPTER III.

THE THEORY OF PSYCHO-PHYSICAL PERCEPTION.

THE theory of psycho-physical perception was the necessary sequence of certain views which I have taken of the nervous system. These views differ very considerably from the ordinarily accepted theories, and, as far as I am aware, are entirely my own.

Most psychologists agree in assuming that the mind is made up of a number of faculties. The phrenologists deserve the credit of having classified these faculties and their classification is a fairly good one. I may say that with phrenology, as expounded by the phrenologists, I totally disagree.

The name applied to these faculties is of secondary importance. When we see that one man is able to perform the most abstruse calculations, whilst another man is hardly able to master the multiplication table, it is evident that there is a difference between the two, which may be admirably expressed by saying that the faculty of calculation is large in the first man and small in the second. We have very little evidence to show where the faculties are situated, whether they are spread over a considerable portion of the brain, or whether they are confined to certain convolutions. I am inclined to hold the latter opinion.

The following are the conclusions which I have come to with regard to the perception of sensations :

:

1. That in the cerebrum there are definite centres having the function of conveying to the mind information respecting impressions which have been conveyed from the external senses.

2. That the seat of memory is situated at a lower portion of the brain than the perceptive centre—that is, at a point between the perceptive centre and sense organ, probably in the optic thalami.

3. That all portions of an impression of sight are connected in the seat of memory; that is to say, the impression is not divided, but the whole exists as a faint impression of that which was at first received. Thus, when we look at a landscape a definite impression is conveyed to the mind, and this impression is remembered as a whole. Different portions of an impression may be accentuated according to the relative size of the perceptive centres for each class of impressions.

4. That the act of perception is the perception of differences in a physical series, the mind as a whole being made aware of these differences through definite perceptive

centres.

5. That the mind is only conscious of the impressions in the seat of memory through the perceptive centres.

6. That defective size of a perceptive centre will prevent the act of perception from being perfectly performed; that is to say, fewer points of difference are evident in the physical series.

For the arguments and facts in support of the above views I must refer the reader to my book on "Memory."

We can only have cognizance of the external world as our senses and faculties inform us of its existence. Any defect, either in the sense-organ or perceptive centre, by preventing the perception of certain classes of sensations, has the same effect as if the physical stimuli

giving rise to the sensations did not exist. It is impossible to explain to a man who has been born blind what sight is; he is unable to appreciate what such a sense can be, and so does not feel the want of it in the same way as a man who has once been possessed of sight.

The fact that defects in the estimation of sensations may be due to some defect in the brain, has been very generally overlooked by those who have been considered authorities. Physicists, as a class, are too much inclined to overlook the personal element when making their investigations. They appear to take for granted that the perceptions of others are similar to those experienced by themselves.

In the perception of a sensation there are the following factors to be taken into consideration :—

1. The physical stimulus.

2. The sense-organ receiving this stimulus.

3. The nerves conveying the effects of the stimulus. 4. The centre of memory receiving the whole impression.

5. The perceptive centres conveying to the mind information concerning individual portions of the impression.

Therefore imperfect perception may be due to a defect in any one or more of the above five factors.

What takes place in normal perception?

Let us take, for example, the perception of size. To illustrate this we must have a physical series. Let us suppose that we have a series of spheres, differing in size from the smallest object capable of being perceived by the naked eye to a very large sphere. Each unit of this series should differ from the adjacent units in a very slight degree. This will approximately represent a physical series. The appearance of this series to any person will be the psycho-physical series for that person. A psycho