514 VISUAL SENSATIONS. [BOOK in. 



ft, these distances ought to correspond to the distances of the retinal 

 vessels v from the sclerotic b on the one hand, and from that part 

 of the retina /fif where visual impressions begin, on the other. H. 

 Miiller found that the distance fii> thus calculated corresponded to 

 the distance of the retinal vessels from the layer of rods and cones. 

 Thus Purkinje's figures prove in the first place that the sensory 

 impulses which form the commencement of visual sensations origi- 

 nate in some part of the retina behind the retinal vessels, i.e. some- 

 where between them and the choroid coat; and H. Miiller's calcula- 

 tions go far to shew that they originate at the most posterior 

 or external part of the retina, viz. the layer of rods and cones. 

 It must be admitted however that H. Miiller's results were not 

 sufficiently exact to allow any great stress to be placed on this 

 argument. 



In the second method of experimenting, the image always moves 

 in the same direction as the light, as it obviously must do. In the 

 first method, where the light enters through the cornea, the image 

 moves in the same direction as the light when the light is moved 

 from right to left, provided the movement does not extend beyond 

 the middle of the cornea, but in the opposite direction to the light 

 when the latter is moved up and down. In Fig. 71, which repre- 

 sents a horizontal section of an eye, if a be moved to a, b will move 

 to /3, the shadow on the retina c to 7, and the image d to 8. If on 

 the other hand a be supposed to move a-bove the plane of the paper, 

 b will move below, in consequence o will move above, and d will 

 appear to move below, i.e. d will sink as a rises. 



FIG. 71. DIAGRAM ILLUSTRATING THIS FORMATION OF PURKINJE'S FIGURES WUKN THE 

 ILLUMINATION is DIRECTED THROUGH THE CORNEA. 



It is desirable in these cases to move the light to and fro, espe- 

 cially in the first method, as the retina soon becomes tired, and the 

 image fades away. Some observers can recognise in the axis of 

 vision a faint shadow corresponding to the edge of the depression 

 of the fovea centralis. 



The retinal vessels may also be rendered visible by looking 

 through a small orifice such as a pin-hole in a card placed close 

 to the eye, at a bright field such as the sky, and moving the 



