TRANSACTIONS OF TIIE SECTIONS. 
549 
in all transparent bodies, and in that great variety of substances 
in which no reflected tints can be discovered, the lost rays are 
detained within the body by absorption. 
On the Undulations excited in the Retina by the Action of lu¬ 
minous Points and Lines. By Sir David Brewster, K.H. 
LL.D. F.R.S. V.P.R.S.E. 
In this communication the author considers a variety of cases 
when light affects other parts of the retina than those on which 
it directly falls,—either by rendering them more or less sensible 
to light and particular colours, or by altering the tints which 
are visible there, or by the excitement of undulations in the 
retina from the illuminated part. The following are the re¬ 
sults of Sir D. Brewster’s experiments on the last of these 
phenomena, as exhibited by the action of luminous points and 
lines. 
1. If we look through a narrow aperture, about the A-th of 
an inch wide, at a bright part of the sky, or at the flame of a 
candle, we shall observe the luminous ground covered with a 
great number of broken parallel lines alternately light and dark. 
These lines are always parallel to the narrow slit, and of course 
change their place as the slit is moved round before the eye. 
Through a number of parallel slits, such as between the teeth 
of a comb, the broken parallel lines are seen more distinctly ; 
and if we give the comb a motion oblique to the direction of 
its teeth, the broken lines become more distinct, though less 
straight than before, and new black lines appear, lying in dif¬ 
ferent directions, as if they were detached portions of a num¬ 
ber of dark ramifications. All these phenomena are seen more 
distinctly when we look at homogeneous light. If we use two 
systems of narrow slits, and cross them at different angles, we 
shall perceive two systems of broken lines crossing each other 
at the same angles ; and if when the lines of the two systems 
are parallel we give one of them a rapid alternating motion per¬ 
pendicular to the direction of its slits, the parallel broken 
fringes are seen with peculiar distinctness. 
2. Phenomena analogous to those now described may be seen 
by looking at a number of parallel black lines drawn upon white 
paper, such as those which represent the sea in an engraved 
map, or by looking at the luminous intervals in a number of pa¬ 
rallel wires seen against the sky. If the eye looks at any of 
these objects steadily and continuously, the black lines soon 
lose their straightness and their parallelism, and inclose hum- 
