TRANSACTIONS OF TI1E SECTIONS. 551 
Upon this principle the author explains the experiments of 
Mr. G. Smith of Fochabers, in which the same object appeared, 
under certain conditions of vision, of different colours to the 
different eyes, the colour observed by the one eye being com¬ 
plementary to that observed by the other. He also refers to 
the same general principle of undulations propagated across 
the retina, for an explanation of the remarkable experiment on 
the eye, first made known by Dr. Purkinje of Breslau. 
In this experiment, if a candle be held before one eye, at 
about a foot distance, and in a direction deviating a little from 
the line of distinct vision,—that eye sees a general mass of red¬ 
dish light around the candle, and in this light, as a ground, 
are seen the ramifying blood-vessels of the retina, the base of 
the optic nerve, and the foramen centrale. Sir D. Brewster 
states it to be the most prevalent opinion, that the light which 
surrounds the candle is reflected back upon the retina, either 
by the inner concave surface of the crystalline lens or of the 
cornea ; and that the objects are, somehow or other, magnified 
by these concave surfaces. His own view of the subject is, that 
the light was propagated from the luminous image of the can¬ 
dle, and that though the retina, in contact with the blood-ves¬ 
sels, is sensible to direct light, it is insensible to propagated 
light, and therefore the blood-vessels must be delineated in 
obscure lines. As there is no retina across the foramen cen¬ 
trale , it will of course appear as a black spot; and, owing to the 
obtuse vision of the optic nerve, it will appear less luminous 
than the surrounding retina. 
After the reading of Sir David Brewsters paper, Mr. 
Wheatstone said, that having been the first person to intro¬ 
duce Purkinje’s beautiful experiment into this country, and hav¬ 
ing repeated it a great number of times under a variety of forms, 
he would take the opportunity of stating a few particulars re¬ 
specting it, which appeared not to be generally known.—The 
experiment succeeds best in a dark room, when, one eye being 
excluded from the light, the flame of a candle is placed by the 
side of the unshaded eye, but so as not to occupy any of the 
central part of the field of view. So long as the flame of the 
candle remains stationary, nothing further occurs than a dimi¬ 
nution of the sensibility of the retina to light; but after the 
flame has been moved upwards and downwards, through a small 
space, for a length of time, varying with the susceptibility of the 
individual on whom the experiment is tried, the phenomenon 
presents itself. The blood-vessels of the retina, with all their 
ramifications, exactly as represented in the engravings of Soem- 
