of Animal Bodies. 303 



when the extremities of two pencils of rays impinge on one and 

 the same papilla, they will not be both as distinctly seen as 

 if one alone fell upon that papilla. We may therefore assert, 

 that we are not very far from the truth in estimating the radius 

 of the image of a physical luminous point on the retina as vary- 

 ing, when distinctly seen, within the limits of the decimals 

 0,001 and 0,0001, the Parisian line being unity. This inference, 

 drawn from the size of the papilla, agrees pretty well, although 

 not exactly, with the results of the optical calculations made 

 by Treviranus, and of experiments performed by Mayer. It 

 is worth noting, as connected with this part of the subject, 

 how much more visible a black object on a white ground is, 

 than a white object on a black ground at the same distance. 

 Thus, a white square whose side equals a Parisian line, when 

 fixed on a black ground, becomes invisible at the distance of 

 about twenty-seven and a half inches when looked at in a weak 

 sunshine ; by ordinary day-light it ceases to be visible at twenty- 

 four inches. Now a black object of the same dimensions, on a 

 white ground, can be seen, under the same circumstances, at 

 double that distance, viz. forty-eight inches. 



Although it is probable that the papillae of the retina, like all 

 ^other nervous papillae, are capable of becoming turgescent when 

 their nervous energy is excited, yet their extreme minuteness 

 prevents this turgescence from being of an amount capable of 

 exerting any appreciable influence in diminishing the distance 

 between the seat of the image on the papillae and the lens. 

 When we look attentively at an object it may become clearer, 

 and may be actually seen with greater distinctness on account of 

 this turgescence. Treviranus thinks that when both eyes are 

 turned to an object with attention, it will be seen with greater 

 distinctness for two other reasons also, — first, because under 

 these circumstances the image falls exactly at the extremity of 

 the optic axis in each eye ; and, secondly, because the iris of 

 each eye adapts itself to its task by regulating the size of the 

 pupil, so as to accommodate it to the distance of the object, and 

 the angle which the extreme rays from the object make with the 

 axis of vision. Treviranus's researches concerning the ultimate 

 structure of muscles, tendons, fibrous membranes, &c. are ex- 

 tremely interesting, and the discoveries he has made concerning 



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