Barrett—On Entoptic Vision. 85 
(3). There are several other well-known entoptic phenomena 
which do not require the aid of the Entoptiscope or a stenopic 
screen for their perception. Such, for example, are the so-called 
Purkinje’s figures: these are the shadows of the capillaries and 
minute vessels of the retina, which can be seen ramifying in all 
directions when a candle is moved to and fro on one side of, and 
a little below, the eye, the observer being in a darkened room, and 
looking straight in front; or they may be seen by moving a per- 
forated screen to and fro in front of a lamp, ora pin-hole aperture 
moved to and fro when the eye is directed to a bright sky ; still 
more easily and vividly can they be seen when a very bright spot 
of light is allowed to fall on the sclerotic coat of the eye, and in 
this case the minute detail of the arborescent form of the vessels 
isweliseen. In any of these ways oblique illumination is obtained, 
and retinal sensitiveness increased, by the successive moving of the 
shadow through the motion of the light or of the head. 
(4). When the eye is directed to a bright sky or cloud, and a 
cobalt-blue glass or gelatine film of the right tint suddenly inter- 
posed, the macula lutea, or yellow spot of the eye, can be seen as 
a small dark patch in the field of view. The best colour to 
interpose is a solution of the blue oxalate of chromium and 
potassium ; but I have found a gelatine film, tinted a purplish 
blue, do very well. The success of the experiment depends on the 
eye not being focussed on the interposed screen, but kept fixed on 
the distant cloud. With a stenopic screen kept in to-and-fro 
motion, and the tinted gelatine interposed, the actual structure of 
the yellow spot can be seen. 
(5). Several observers have noticed that when the opened 
fingers are moved to and fro in front of the eye, or, still better, 
the above coloured screen interposed, the eye being directed 
to a bright cloud, a remarkable movement like the circulation 
of the blood is perceived in the vessels of the retina. Vierordt, 
in 1856, first drew attention to this. Prof. Ogden Rood inde- 
pendently noticed it, and published an interesting paper on the 
1 Writers on physiological optics appear to have overlooked the fact that Sir C. 
Wheatstone gave the first explanation of this phenomenon (vide British Association 
Report, 1832, p. 551). 
