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shadow thrown back upon that surface, through reflexion from some 

 tunic at the distance d without it, we should have 



And some slight modification of this equation would meet the case 

 of a parallactic deviation by a second reflexion, should a vessel lie 

 either a little without or within the sentient surface. 



The parallaxes here suggested would all take place in the direc- 

 tion of those actually witnessed in Purkinje's experiment, and the 

 calculated values of d would so far approach each other from the 

 same values of a and ft (since /3 is small), that we ought not to rest 

 satisfied with the fact of these values agreeing well with the simple 

 conception of the sentient surface lying a little without the vascular 

 plexus ; especially as there are two supplementary versions of the 

 vascular phantom rendered manifest by the experiment, one very 

 notable one to which attention is directed and that were the 

 shadows of the vessels displayed by a second reflexion, as imagined 

 in the third equation, there would be more than one version of them ; 

 as, moreover, a dark figure of the foramen centrale is rendered visible 

 at the same time. However, these other versions of the figure are 

 eliminated by being traced to other sources, and with H. Miiller, the 

 central dark spot is treated as the shadow of the wall of the foramen 

 centrale ; so that the sentient surface must be without the brim of 

 the foramen. 



The essay, with as much of method as is available, now passes on 

 in quest of the causes of other spectral phenomena, in the production 

 of which light is not an agent. It cites Young's observation of the 

 images of objects that press from the outside of the eye upon the 

 sclerotic coat, being seen by flexure of the retina along their outline ; 

 notices, as of this type, the circles of light seen at the bases of the 

 optic nerves on turning the eyes sharply in their sockets ; touches 

 upon the colours displayed in such experiments ; and points out how 

 that wherever the retina is so compressed as to evince quasi-lights, 

 it comparatively or entirely fails to render us acquainted with the 

 existence of luminous objects. It then explains how the retina is 

 flexed and compressed by the action of the orbital muscles, always, 



