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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



described on p. 767. Another argument in favor of this view is found in the 

 correspondence between the size of the smallest visible images on the retina and 

 the diameter of the rods and cones. A double star can be recognized as double 

 by the normal eye when the distance between the components corresponds to 

 a visual angle of 60". Two white lines on a black ground are seen to be dis- 

 tinct when the distance between them subtends a visual angle of 64"-73". 

 These angles correspond to a retinal image of 0.0044, 0.0046, and 0.0053 mil- 

 limeter. Now, the diameter of the cones in the macula lutea, as determined 

 by Kolliker, is 0.00450.0055 millimeter, a size which agrees well with the 

 hypothesis that each cone when stimulated can produce a special sensation of 

 light distinguishable from those caused by the stimulation of the neighboring 

 cones. The existence of the so-called blind spot in the retina at the point of 

 entrance of the optic nerve is sometimes regarded as evidence of the light- 

 perceiving function of the rods and cones, but as the other layers of the retina, 

 as well as the rods and cones, are absent at this point, and the retina here 

 consists solely of nerve-fibres, it is evident that the presence of the blind spot 



FIG. 233. To demonstrate the blind spot. 



only proves that the optic nerve-fibres are insensible to light. Figure 233 is 

 intended to demonstrate this insensibility. For this purpose it should be held 

 at a distance of about 23 centimeters from the eyes (i. e. about 3.5 times the dis- 

 tance between the cross and the round spot). If the left eye be closed and the 

 right eye fixed upon the cross, the round spot will disappear from view, though 

 it will become visible if the eye be directed either to the right or to the left of 

 the cross, or if the figure be held either a greater or a less distance from the 

 eye. The size and shape of the blind spot may readily be determined as 

 follows : Fix the eye upon a definite point marked upon a sheet of white 

 paper. Bring the black point of a lead pencil (which, except the point, has 

 been painted white or covered with white paper) into the invisible portion of 

 the field of vision and carry it outward in any direction until it becomes vis- 

 ible. Mark upon the paper the point 

 at which it just begins to be seen, and 

 by repeating the process in as many 

 different directions as possible the out- 

 line of the blind spot may be marked 

 out. Figure 234 shows the shape of 

 the blind spot determined by Helm- 

 holtz in his own right eye, a being 

 the point of fixation of the eye, and 

 the line A B being one-third of the 

 distance between the eye and the paper. The irregularities of outline, as at 



A\. 



FIG. 234. Form of the blind spot (Helmholtz). 



