membrane in such a manner as to penetrate from the side of the 

 hyaloid membrane outwards from the eye's centre, deeper as they 

 approach the punctum aureum. They may be called into view by 

 any pencil of rays that is in the act of sweeping over the retina. 

 In the well-known experiment of Purkinje, of waving a candle- 

 flame before the eye, the radiating point is the image of the flame 

 at the back of the eye. In this experiment the vessels display a 

 remarkable parallactic gliding over the visual field, first observed by 

 Gudden, in 1849 ; but it was left for H. Miiller, a few years after, 

 to point out the cause of the phenomenon, and to calculate from 

 entoptical observations the distance of the vessels from the "per- 

 ceiving membrane " lying without them, required to account for the 

 parallax. This essay adopts his hypothesis, but supports it by in- 

 dependent observations, and substitutes another mode of calculating 

 the said distance. 



The flame is made to pause successively, on opposite sides of the 

 vessel to be observed, in an ocular meridional plane, or that of some 

 great circle, so that the shadow may be seen to deviate equally, twice 

 in one plane, from the retinal radius that passes through it, and this 

 whole angle, ft, is noted, as well as that, a, between the two positions 

 of the flame, as viewed from the eye's centre. Then, if d be the 

 perpendicular distance of the vessel from a sentient surface whose 

 radius is r, it is found that 



If we imagine a dark spot without the sentient surface, as the 

 pigment of the choroid, visible through the retina at the foramen 

 centrale, to simulate a shadow by being seen, through a second 

 reflexion of the rays already radiating by reflexion at the flame's 

 image on the eye's coats, by a sentient surface lying within it ; then 

 if d be the distance of the dark spot from such a surface, we shall 

 have 



^. 



I COS 4 a J 



If a vessel imbedded in the sentient surface were to have its 



