SENSE OF SIGHT. 445 



creases in the anterior part ; thus it is 150 times less at the 

 equatorial plane of the eye than in the yellow spot or 

 macula lutea ; thus, if we place two wires very close together, 

 but with sufficient space between them to enable us to dis- 

 tinguish one from the other, and then so direct the eye that 

 their image shall fall, first upon the yellow spot, and then upon 

 the great circle of the eye, we shall find, in the latter case, 

 that the wires to seem distinct must be placed at a distance 

 from each other 150 times greater than when they are painted 

 upon the yellow spot. This experiment is exactly similar to 

 that made in regard to the distance between the points of 

 the dividers, by means of which we estimated the degree of 

 sensibility of the skin (see p. 398). 



The yellow spot is, therefore, the principal seat of distinct 

 vision. We make use chiefly of this in order to see clearly, 

 and all the movements of the eyeball are designed to bring 

 the image of the objects observed to this extremely sensitive 

 point in the eye. The entire surface of the retina is about 

 15 square centimetres, while the surface of the macula lutea 

 is only 1 millimetre ; we therefore make use of only -gfo -part 

 of the surface of the retina for the purpose of distinct vision. 

 Thus, in reading, we see distinctly only two or three words 

 at a time, their image would fall directly on the yellow spot ; 

 and the eye must pass over the whole line in order to read 

 it ; in other words, it must bring the image of every single 

 word to the sensitive point. In order to decide exactly what 

 is the number of letters, or the extent of surface, painted 

 on the retina, the eyes are fixed, in a dark room, upon the 

 page of a book ; the number of letters which can be seen by a 

 flash of lightning or by an electric spark, is then counted, and 

 the dimensions calculated. Starting from this datum, the 

 known dimensions of the yellow spot may be calculated. 



Having observed the various degrees of sensibility of the 

 different parts of the retina, we must now examine the sub- 

 stance of this membrane, and see whether, among its numer- 

 ous layers, there is not one which is peculiarly sensitive, and 

 containing an element which is essentially susceptible to the 

 influence of light. A simple experiment will supply us with 

 a sufficiently satisfactory answer to this inquiry : this experi- 

 ment is known by the name of Purkinje's vascular tree, and 

 consists in the perception of the vessels, or, rather, of the 

 shadow of the vessels of the retina itself. These vessels, 

 which are situated in the anterior layers of the retina, always 

 cast their shadow upon the posterior layers of this meinbrare, 



