128 Scientific Proceedings, Royal Dublin Society. 
the layer of rods and cones.’! I have been unable to verify this 
conclusion, as the experiment is one of considerable difficulty, not 
only because of the elusive character of the observations, but also 
owing to the uncertainty of determining the distance between the 
retinal vessels and the points on the sclerotic where the light 
impinges, for the length of this path will vary in different eyes. 
If Purkinje’s figures are formed by a candle moved to and fro 
near the cornea, then fig. 2 illustrates, according to H. Muller, how 
they are probably produced, a horizontal section of the eye 
being shown. If the candle be at the point a, the illuminated 
spot on the retina 6 will reflect light, and throw a shadow of some 
Fie. 2. 
intervening vessel upon the retina at the point c; this shadow will 
be seen projected from the eye to some external position d. Ii 
now the candle be moved from a to a, the path of the rays will 
be indieated by [3 and y, and the projected image will be seen 
at 6. That is to say, the projected image moves towards the centre 
of the field of view as the candle moves from it, both moving in 
the same direction ; but if the candle be raised, the image will be 
depressed, the image now moving in the opposite direction to that 
of the candle.’ 
1 Sir M. Foster’s ‘‘ Text-book on Physiology,”’ 5th edition, Part iv., p. 1250. 
2 This will be clear by supposing the candle a raised above the plane of the paper ; 
B will then be depressed, whilst y will be raised, and the projected image 6 will be 
accordingly depressed. In the lateral movement of the candle, when the light passes 
the pupil of the eye, a change in the direction of the projected image occurs, its motion 
being now opposite to that of the candle. The projected image, in fact, describes a 
path somewhat like the letter C turned sideways, thus: ©. 
