534 THE NERVOUS SYSTEM. 



and yet an abundance of details are distinctly perceptible within this 

 space. The extreme limit of approximation at which two points may 

 be distinguished from each other has been examined by the observation 

 of fixed stars, and by that of parallel threads of the spider's web, or of 

 fine wires, placed at known distances from each other.* These exami- 

 nations show that, for average well-formed eyes, the smallest visual 

 angle, at which adjacent points can be distinguished, is from 60 to 73 

 seconds ; corresponding to a distance upon the retina of from 4 to 5 

 mmm. According to Schultze, the diameter of the retinal cones, at the 

 fovea centralis, is from 3 to 3.5 mmm. ; and if two beams of light were 

 separated at the retina by a less distance than this, they might fall 

 upon the same cone, and consequently excite the same connecting fibre 

 in the adjacent layer. If the diameter of the cones be the element 

 which determines the acuteness of vision, two luminous points, to be 

 distinctly perceptible, must be separated upon the retina by a distance 

 of at least 3 mmm., and must have a visual angle with each other of 

 at least 42 seconds. In astronomical observations, it is found that two 

 stars can never be separately distinguished by the eye, unless their 

 angular distance from each other is equal to 30 seconds; and very 

 seldom, unless it be as great as 60 seconds. These measurements are 

 hardly sufficient to decide the question ; since there has never been an 

 opportunity of examining the size of the retinal elements in an eye, of 

 which the acuteness of vision had been previously tested. But they 

 are enough to indicate a probable connection between the ifenute 

 structure of the retina and the limit of its sensibility to separate im- 

 pressions. 



The Retinal Red and its alteration by light. The retina, as usually 

 extracted from the eye of a recently killed animal, is colorless or slightly 

 opaline. But in its normal condition in the living eye, or if extracted 

 without exposure to light, it is of a purple-red hue, due to a transparent 

 coloring matter in its external or posterior layer. This color, the so- 

 called "retinal red," first discovered by Boll,f has been more fully 

 investigated by Kiihne.J It is seated exclusively in the rods of the 

 retina, and is consequently most distinctly marked where these ele- 

 ments are most abundant. The cones, on the other hand, are color- 

 less. At the macula lutea, accordingly, where the cones preponderate 

 over the rods, the reddish tint disappears ; and it is entirely absent at 

 the fovea centralis, where the membrane consists only of cones and 

 their appendages. Elsewhere, it extends over the retina to within 

 three or four millimetres of the ora serrata, where it terminates by a 

 tolerably well-defined limit. 



* Helmholtz, Optique Physiologique. Paris, 1867, p. 292. 



f Monatsberichte der konigliche Preussischen Akademie der Wissenschaften. 

 Jahre 1876. Berlin, 1877, p. 783. 



J Untersuchungen aus dem Physiologischen Institute. Heidelberg, 1877. Heft 

 1, 2, 3. 



