126 Scientific Proceedings, Royal Dublin Society. 
after having been detained by the pressure which is now inter- 
mittent.”! I am not aware that anyone has hitherto drawn 
attention to this early experiment of Young and his remarkably 
correct explanation of what he observed. 
A much better and more convenient method of vividly seeing 
Purkinje’s figures is to allow a brilliant point of light to fall on 
the sclerotic coat near the cornea. For this purpose it is best to 
use a pencil of rays from the electric-light or lime-light, though 
any bright source of light will do if the rays are concentrated by 
a short focus lens on to the sclerotic. The observer should be 
seated in a darkened room ; and an assistant should manipulate the 
lens so that the beam of light falls at the right spot where the 
sclerotic is thin, as it is around the cornea. In this case the 
shadows of the retinal vessels are thrown directly on the retina by 
the glare of light from the illuminated point of the sclerotic. The 
appearance that suddenly starts into view has been admirably 
depicted by Mr. Ledwidge, and is shown in fig. 1, Plate VIII. 
In my own case I see the finer capillaries, after branching, again 
uniting into a single larger vessel. This occurs in all the smaller 
ramifications seen in the field of view; but Mr. Ledwidge has 
drawn what he has seen near the fovea, where the finer capillaries 
do not appear to unite again. 
The field of view is more extensive than is shown on the plate, 
a portion only of the original coloured drawing having been 
reproduced in black and white. The background is of a bright 
yellow or orange colour; and one portion of the field appears to 
contain few or no vessels, and has in its centre a peculiar granular 
or cellular structure. This Mr. Ledwidge will describe in an 
appended Note, as he saw it more clearly than I did. Helmholtz 
refers to this particular appearance and compares it to “‘shagreen 
leather.’ If the head be moved slightly in one direction, the 
magnified projected shadows of the vessels, seen, of course, inverted, 
will correspondingly be displaced; but the peculiar granular spot 
will be less displaced than the shadow of the vessels, showing 
that it is nearer thé seat of vision.” 
1 Philosophical Transactions, 1793, vol. Ixxxiii., p. 169. Young’s Collected Works, 
vol. i., p. 10. 
2 Mr. Ledwidge found this smaller displacement to be in the opposite direction to 
the general network of vessels. (See his Note at end.) 
