Many persons are not aware of the extent of the field of view which the eye embraces. Vertically it takes in about 100°, whilst horizontally it will take in some 145°, more or less. The field is smaller on the nasal than on the temporal side. When both eyes are used, the combined field of view is larger horizontally, being about 180°. The field of view which is common to both eyes is roughly a circle of about 90°. There is, however, a marked difference in the distinctness with which objects are perceived in the different parts of field of view. On the fovea centralis two dots placed so as to subtend an angle of 60″ will be perceived as double. That is to say, if a piece of paper, on which are two dots 1/30 of an inch apart, be placed 10 feet away from the observer, these dots will be seen as separated, whilst dots (in this case they should be black and of good dimensions) placed half-an-inch apart would still appear as one if viewed at the same distance near the periphery of the retina. In the yellow spot the distance apart of the cones is such that they subtend about the same angle as the dots when they are seen separate, viz., about 60″; that is, they are about 16/100000 of an inch apart, and hence may have something to say to the limit of separation. The field for the perception of colour is different to that for light.

The diagrams ([Fig. 3]) will show the fields in a satisfactory manner. The concentric circles are supposed to be circles lying on the retina corresponding to parallels of latitude on a globe, and are not, therefore, equi-distant when seen in projection. To make these circles it must be imagined that we have a bowl, in the middle of which is a thin rod standing upright and passing through the centre, and another rod attached to it at the centre of the sphere of exactly the length of the radius. If this last arm be opened to make an angle of 5° with the fixed rod, and be twisted round like the leg of a compass against the bowl, it will make a circle, the projection of which will give the innermost circle of the diagram; if opened to 10° it will give the next circle, and so on for every subsequent 10°. The lines passing through the centre are 30° from one another, the line stretching from 360° to 180° being a line supposed to be vertical. By means of an instrument called the perimeter, the field of vision for each eye can be measured. With its aid any small object can be made to fall on any part of the retina by directing the axis of the eye to a fixed point and moving the object along one of the diameters. Suppose we wish to ascertain the field for a white object, a small white disc is moved, say, along the horizontal line, and the angles at which the retina just no longer sees it are noted. This gives two points in the field, and they are plotted on the chart—in [Fig. 3] one touches the outside circle, and the other is at an angle of about 65°. The field of vision is next tested along another line, say 300° to 120°, and other points noted and marked on the chart. When the whole circle has been examined, the various points are joined together, and we have the boundary of vision for a white object. The boundaries of the colour perception for (say) small red and green discs are found in the same way. The former is depicted in the left-hand chart and gives the field for the right eye, and the latter with that for white in the right-hand chart for the same eye. It will be noticed that two boundaries are given, one taken at mid-day and the other at 6 p.m. The brighter the colour, the larger is the boundary in both cases, showing that the field of colour vision varies according to the illumination. Now it is difficult from this method of experimenting to determine whether the fields for different colours are the same or differ in extent, as we have no information as to whether the colours themselves which were used were physiologically equal. The only way by which this can be satisfactorily determined is by using spectrum colours each of known brightness and area. (Some preliminary experiments made by myself regarding the colour fields will be found in the appendix, and will be referred to later.) It must not be thought that the various colour boundaries mark the limit at which light is perceived, but only the limit at which colour is seen; outside the boundaries the objects appear of a nondescript colour, to which we shall by-and-by call attention. The yellow spot lies within the circle of 5°, and the blind spot on which no sensation of light is stimulated is shown by the black dot about 15° away from the centre.

I have only attempted to sketch, in unphysiological language, the primary apparatus with which our experiments in colour have perforce to be made.

CHAPTER II.

It will be seen, then, that in measuring colour or light several circumstances have to be taken into account. These are not simple, and require differentiating one from another before the results of colour measures can be finally laid down as correct, or as being held to be applicable to all cases.

We must naturally ask, what is colour? The answer I should like to pass over entirely. It can only be described as a sensation, just as we should describe touch as a sensation. It has, however, one advantage over most sensations, in that it is a sensation which can be submitted to empyric measurement. The question whether certain phenomena, such as the colours produced by simultaneous contrast, are subjective or real, does not require answering for the purpose that we have in view, but the results recorded may probably help to throw light on it. Colour is an impression caused by the stimulation in the eye of some apparatus, that lies near the outer wall of the retina, the effect of the stimulation being conveyed by the optic nerve to the brain. If this apparatus be complicated by being made up of distinct parts, each of which transmits its own kind of impression to the brain, it is not only quite possible, but more than probable, that when one part is absent or injured the particular impression for which it is responsible will be lacking, and that the sum of the impressions due to the remainder will be unlike that perceived when they are all working together.

In every investigation, whether it be in physical or in any other branch of science, it is better to work up from the simple to the more complicated; and acting on this plan, it is better to commence experimenting with simple rather than with complex colours, though they may apparently produce precisely the same sensations. I shall, with this in view, devote most of the remaining part of this chapter to some necessary experiments with simple colours. The simple colours are those of the spectrum, and are the result of motion in the ether, which pervades all space. The motion is in the form of undulations or waves, and each colour is due to a series of these waves, which have a definite length. Thus, 6562 ten-millionths of a millimetre produces to most of us a red colour in the spectrum (see Plate I.), occupying the position indicated by a black line known as the C line in the solar spectrum.

A table of wave-lengths of certain lines in the solar spectrum is given below:—

TABLE OF WAVE-LENGTHS IN TEN-MILLIONTHS OF A MILLIMETRE.

B, deep red6866
Lithium, cherry red6705
C, red6562
D, orange5892
E, green5269
b, green5183
F, bluish green4861
Lithium, blue4603
G, violet4307
H, extreme violet3968