OF THE RETTHA TO LIGHT AND COLOUR, 
171 
Table VIII. 
Aperture. 
"W idth of 
Readings. 
Logs from 
diagram. 
powers of 2. 
Degrees. 
Logs. 
Circular disc, ‘Qt incli diatn. 
- -09 
234 
1-98 
1-98 
Square, ‘84 inch side. 
Rectangle, 1‘68 x ‘42 inch. 
- ‘25 
216 
214 
2T5 
- 1-25 
152 
2-69 
2‘65 
Rectangle, ‘84 x ‘42 inch. 
- 1-2.5 
154 
2-68 
2‘65 
Remarks on this table seem unnecessary, as they so jjlainly indicate the guiding 
factor in the extinction. 
This, perhaps, is one of the most curious results that have been obtained, for it is 
hard to conceive that the area of the retina impressed should not be a factor. The 
experiments clearly show that the estimate of small intensities of light by their 
effect on the light-perceiving apparatus is not a simple matter. The extinction of 
comparatively larger areas of light is most instructive. The light from a square, or 
a disc, or an oblong, just before extinction, is a fuzzy patch of grey, and appears 
finally to depart almost as a point. This can scarcely account for the smallest width 
of an illuminated surface determining the intensity of the light just not visible ; but 
it tells us that the light is still exercising some kind of stimulus on the apparatus, 
even when all sensation of light is gone from the outer portions. The fact 
th?T the disappearance of the image takes place in the same manner, whether 
viewed centrally or excentrically, tells us that this has nothing to do with the 
yellow spot or fovea, but is probably due to a radiation of sensation (if it may be so 
called) in every direction on the retinal surface. Supposing some part of the 
stimulus impressed on one retinal element did radiate in all directions over the 
surface of the retina, the effect would be greatest in the immediate neighbourhood, 
and would be inappreciable at a small distance, but the influence exerted upon an 
adjacent element might depend not only on its distance, but also upon whether 
it was or was not itself exerted independently. Following the matter out further, 
we should eventually arrive at the centre of an area, being the part which was the 
recipient of the greatest amount of the radiated stimuli, and consequently that would 
be the last to disappear. With a slit aperture, the slit is visible till extinction is 
very nearly executed, but it Anally merges into a fuzzy spot at the moment before 
it finally fails to make any impression of light. 
9. Extinction of Light Excentrically. 
A further investigation into the extinction of light at different angular distances 
from the centre of the eye was attempted. The experiments are of a very difticult 
nature, and it recjuires long practice to enable a satisfactory seiies to be made. 
z 2 
