OF THE RETINA TO LIGHT AND COLOUR. 
161 
the insertion of diaphragms, d, in front of the glass. The eye-piece, E, was at the 
opposite end of the box, as shown. Outside the box was a mirror, M, enclosed in a 
frame, as shown. The slit, S, in the spectrum and the collecting lens, L, together 
with the wedge, W, are shown in the figure. H is the handle used to move the 
annulus round its axis. This form of box only admits light through the end ; there 
is no reflected light in the inside. The one thing necessary is to secure a good 
scattering of light by the ground glass, so that the direct light is inappreciable com¬ 
pared with that scattered, a desideratum which is obtained by using glass ground 
on each surface. The box in which M is fixed is blackened, or lined with black 
velvet, and M itself can be either a silvered or plain glass. In the old arrangement 
the light entered from the side and by reflection, and after passage through ground 
glass illuminated a white disc at the end of the box. When the disc was of fair size 
any reflections from the black interior were extinguished long before the light from 
the disc itself vanished, and hence no inconvenience was felt from the presence of the 
light from the black interior, which may be taken as about g-j of that reflected from 
the disc. If, however, the area of the white disc is very much diminished, the 
illumination, as will be seen presently, may be as much as 100 times greater than 
on the larger disc and yet be invisible, and then the reflection from the interior 
of the box would be visible after the extinction of Imht from the small disc was 
O 
completed. For this reason the new form of extinction box was designed. 
5. Sensitiveness oj the Retina to the Spectrum Colours U'lth Varying Sizes of Image, 
The first series of experiments to be described, though not first in order of making, 
was to ascertain the sensitiveness of the eye to different sizes of image at the point 
of extinction. It has long been known that equally illuminated images on the 
retina, but of varying size, produced different sensations of brightness, but I am 
unaware of any exact measures of the difference. These I propose to give, and trust 
that they will be of use in studying the physiology of the eye. It was, of cou)-se, 
impracticable to use many eyes in this research, and I have been content with the 
observations made by myself and by my assistant, Mr. Bradfield, more especially as 
they were made Cjuite sejDarately, at different times, and are quite confirmatory of 
each other. The size of the image on the retina is best expressed in tlie angular 
measure which the object subtends outside the eye, and this plan I have adopted 
throughout. In Part III., “ Colour Photometry,” the extinction curves showed the 
absolute values of the light of the various parts of the spectrum, when just failing to 
impress the retina, in terms of a candle, and a change in the scale of ordinates several 
times in the same figure was necessitated. When the results of my first set of 
experiments were plotted with the angles of the annulus at which extinction occurred 
VOL. CXC.—A. 
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