550 
SECOND REPORT - 1832. 
nous spaces somewhat like the links of a number of broken 
chains. When this change takes place, the eye which sees it 
experiences a good deal of uneasiness,—an effect which is com¬ 
municated also to the eve which is shut. When this dazzling 
effect takes place, the luminous spaces between the broken 
lines become coloured, some with yellow and others with green 
and blue light. 
The phenomena produced in these two experiments are off 
viously owing to rectilineal undulations propagated across the 
retina ; and the interference and crossing of the undulations, 
by which the dark lines are broken into detached portions, and 
by which the colours are produced, arise from the unsteadiness 
of the head or the hand, which causes a want of parallelism in 
the successive undulations. 
3. The action of small and bright points of light upon the 
retina produces phenomena of a very interesting kind. If we 
look at the sun through a small aperture at a great distance 
from the eye, or if we look at the diminutive image of the sun 
formed by a convex lens or a concave mirror, or seen in a con¬ 
vex surface, the light which falls upon the retina does not form 
a sharp and definite image of the luminous point, but it sends 
out in all directions an infinity of radiations, covering in some 
cases almost the whole retina. These radiations are extremely 
ml 
bright, and are accompanied in some cases by mottled colours of 
great variety and beauty. The bright point of light propa¬ 
gates around it circular undulations, which are broken and co¬ 
loured by interference, and which, being in constant motion from 
the centre of the retina in all directions, occasion the radiations 
which have been mentioned. 
4. If we look at the radiant image just described through a 
narrow aperture, a very singular effect is produced. A vortex 
of circular rays appears on each side of the radiant point, and 
the rays have a rapid whirling motion. The line joining the 
centres of the two vortices is always perpendicular to the nar¬ 
row aperture. This remarkable configuration of the rays is 
evidently produced by the union of a system of parallel undu¬ 
lations with a system of circular ones, the intersections of the 
parallel fringes and the diverging radiations forming the cir¬ 
cular rays, as in the case of ordinary caustics. 
The preceding phasnomena, continues the author, whatever 
be their true cause, clearly prove that light incident upon the 
retina exerts an action on parts of it upon which it does not di¬ 
rectly fall, and that the same action renders other parts of the 
retina insensible to the light which actually falls upon these 
parts. 
