446 ORGANS OF THE SENSES. 



and we can only suppose that it is the force of habit which 

 prevents our being generally conscious of this shadow ; the 

 question was, whether it could not be rendered visible by 

 being thrown, artificially, upon some other part of the eye. 

 This was done in the following manner: 1 the person making 

 the experiment looks r.t a dark obscurity, while a lighted 

 candle is placed either below, or at the side of his eye (Fig. 

 122); the rays proceeding from this light (B) will be con- 

 centrated by the crystalline lens upon a point very much to 

 one side of the retina, the source of light (the candle) being 

 very far beyond the visual centre. The image of the candle 

 on the retina itself constitutes an interior source of light (B') 

 which is sufficiently strong to carry a considerable quantity of 

 light into the vitreous body. It is plain that, under the in- 

 fluence of this light, the vessels of the retina (C and D) will 

 cast their shadow upon the posterior 

 layers of the retina, not, however, in the 

 usual portions (that is, 0' and D'). The 

 shadow will be displaced, and thrown 

 upon the side opposite to that of the 

 source of light in the retina, which is on 

 the same side as the candle (the original 

 source of light). The field of vision 

 being then illuminated by a light of a 

 Fig - 122- Experiment yellowish red, a network of dark-colored 



byPurkinje.* * . . 



vessels is seen to appear, exactly resem- 

 bling the vessels of the retina, as sketched from an anatomical 

 preparation. (Vascular tree of Purkinje.} 



The posterior layers of the retina are, therefore, sensitive 

 to light; but this experiment shows us which of these layers 

 is especially sensitive. By means of a mathematical process 

 which we cannot now describe, and judging by the move- 

 ments of the shadows of the vessels when the source of light 

 is displaced, or, in other words, by the apparent magnitude 

 of the movement produced in the field of vision by the vas- 

 cular tree, Helmholtz has inferred that the layer which receives 



1 See Helmholtz, " Optique Physiologique." Traduct. frang. 

 par E. Javal et Th. Klein. Paris, 1867, p. 214. 



* B, A candle placed at the side of the eye, that is, as much to the side of 

 the centre of the cornea as possible. IV, Interior luminous source, iormed by 

 the rays of light concentrated by the crystalline lens upon the extreme lateral 

 portion of the eye. CD, Two vessels of the retina (the size of the retina is here 

 greatly exaggerated). The shadow of these two vessels is seen as if projected 

 at D' and CT. 



