124 Scientific Proceedings, Royal Dublin Society. 
The horizontal and vertical diameters of the blind spot, in 
the three cases, were as follows in mm. :— 
Mr. W., hor. dia. 1°6, ver. dia. 1:8 m.m. 
Mr. L., ry) 1°5, 29 20 9 
BHO IB ¥ lee ite OO 
The remarkable vertical length of the blind spot in my own case 
surprised me; the results are shown on Plate VII. ‘The foregoing 
results were obtained before I was aware of the measurements 
made by Continental observers, to which I have already referred. 
The agreement is remarkable, and establishes the value of k which 
we have taken. 
4, PurKIngJE’s FIGURES. 
In 1819 the eminent German physiologist Purkinje noticed that 
when a candle, held at the side of the head, was moved up and 
down in an otherwise darkened room, an arborescent figure was 
seen in the field of vision of the observer, the figure being 
projected from the eye adjacent to the moving candle. The 
appearance exactly resembles the branching network of vessels on 
the retina, and Purkinje rightly concluded that it was caused by 
the shadow of those vessels thrown on the retina by the glare of 
the candle. Sir Chas. Wheatstone, in a paper read before the 
British Association in 1832,! showed that, instead of a moving 
candle, a revolving disc with a circular aperture might be employed, 
the moving disc being placed between the eye and a source of 
light.’ 
Wheatstone also first explained why the appearance was only 
seen when the candle or the disc was in motion: he showed 
that to perceive any very small differences of illumination the 
retina must be extremely sensitive, and that this sensitiveness 
rapidly decreases when the shadow is stationary owing to the same 
parts of the retina being stimulated. But when the shadow is 
1 British Association Reports, 1831-32, p. 551. Wheatstone’s Collected Papers, 
p. 2238. 
2 A simple method, which some find successful, is to make a large pin-hole ina 
piece of card, and, looking at a bright sky or brightly illuminated surface, move the 
card, which should be close to the eye, from side to side, or up and down; the finer 
retinal vessels can thus be seen. 
