THE ACTION OF THE RETINA. 451 



that all photographic effects are the effects of a high temperature. 

 "The impingement of a ray of light, on a point, raises the tempera- 

 ture of that point to the same degree as that possessed by the source 

 from which the ray comes, but an immediate descent takes place through 

 conduction to the neighboring particles. This conducted heat, by 

 reason of its indefinitely lower intensity, ceases to have any chemical 

 effect, and hence photographic images are perfectly sharp on their 

 edges. It may be demonstrated that the same thing takes place in 

 vision, arid in this respect it might almost be said that vision is a pho- 

 tographic effect, the receiving surface being a mathematical superficies, 

 acting under the preceding condition. All objects will, therefore, be 

 definite and sharply defined upon it, nor can there be anything like a 

 lateral spreading. If vision took place in the retina, as a receiving 

 medium, all objects would be nebulous on the edges." 



The large amount of blood distributed to the choroid coat, and its 

 proximity to the retina, also render it probable that the former, arid 

 not the retina, becomes the receiving screen. Supposing the retina 

 typically composed of three transparent layers, one of tubules, one of 

 vesicles, and one of granules, the luminous beams pass through them 

 just as through the atmosphere. The vesicular layer undergoes rapid 

 metamorphosis, the effect of which is transmitted by the tubular layer, 

 while in the granular layer lie the germs whence the vesicular layer 

 can be easily redeveloped. But the waste of the vesicular layer can- 

 not occur except under the oxidizing influence of the arterial blood, 

 nor can the nutrition of the granular layer be accomplished without a 

 similar condition. Both are supplied by the construction and proxim- 

 ity of the choroid coat ; and the analogy to the skin, both as to waste 

 and restoration, is still kept up. 



F; G. S.] 



The existence of the so-called blind spot in the retina, does not 

 produce any obvious defect of vision, when both eyes are used, be- 

 cause the image of an object falling on the optic eminence of one eve, 

 naturally falls elsewhere, and on a sensitive part of the retina, in the 

 other eye ; and so the blank in vision is filled. In the use of one eye, 

 the defect is partly remedied by the phenomena known as irradiation, 

 hereafter to be explained, and partly by our experience and knowl- 

 edge of the actual forms of objects ; furthermore, any impairment of 

 sight, dependent on the existence of the blind spot, occurs beyond the 

 area of distinct vision, and therefore attracts less attention than it 

 would otherwise do, and is more easily corrected by experience, or by 

 the effects of irradiation. If the fibres of the optic nerve, at the 

 blind spot, were directly sensitive to light, then they would receive at 

 least two impressions one from their retinal extremities, and the 

 other from the light falling on the optic eminence ; such a condition 

 would lead to confused vision. 



Miiller and other physiologists, however, have denied that the retina 

 at the point of entrance of the optic nerve is wholly insensible to 

 light. They believe that the excitability of the retina is there pecu- 

 liarly diminished, but that it exhibits, in a marked degree, the phe- 



