312 PHYSIOLOGY CHAP. 



on the retina and are perceived as dark spots in the field of vision. 

 If the light is moved in different directions in the same plane, the 

 dark spots in the visual field also move, in proportion to their 

 distance from the retina. 



The larger opacities, which are never absent under physio- 

 logical conditions, are cast by the rich vascular network formed 

 in the inner layers of the retina by the central vein and artery. 

 As this vascular network lies in front of the external sensitive 

 layer of the retina, it would seem natural that we should normally 

 be aware of the shadow projected by it. But under ordinary 

 conditions this is not so. Helmholtz explains this fact by assum- 

 ing that in the parts of the retina that are shadowed by vessels 

 retinal sensibility is greater and its excitability less exhausted in 

 comparison with the other parts that are not shaded and are there- 

 fore more influenced by light. 



But when the eye is illuminated so that the shadows of the 

 vessels are displaced and projected obliquely on to spots that are 

 not shaded in the ordinary passage of the light-rays, the vascular 

 shadows are at once recognised, as Purkinje (1819) first observed. 

 The simplest method of producing this effect is to illuminate 

 one of the eyeballs, which is directed on a wall of the darkened 

 room, and accommodated for distant vision, obliquely by a candle 

 held near the temple, so that the light enters the eye through the 

 sclerotic. After moving the flame up and down a few times, the 

 inverted image of the whole vascular network of the retina is pro- 

 jected, highly magnified, upon the wall. 



Under special conditions it is also possible to see the movement 

 of the blood corpuscles in the capillaries of the retina (Boissier) 

 as on looking through a blue glass at the sun, with accommoda- 

 tion relaxed. Shining dots, like sparks, are seen to move in figures 

 up and down certain set paths. According to His this phenomenon 

 depends on the concentrated light projected upon the outer layer 

 of the retina by the erythrocytes circulating in the capillaries of 

 its middle layer. If a red glass is substituted for the blue, the 

 effect disappears owing to the less absorption of this light by 

 haemoglobin (Abelsdorff and Nagel). 



VIII. We have seen that the contraction of the sphincter of 

 the iris is associated with that of the ciliary muscle in accommoda- 

 tion. The pupil, like the diaphragm of a photographic camera or 

 a telescope, intercepts the homocentric peripheral rays of a cone 

 of light from the fixed point, and thus reduces chromatic and 

 spherical aberration : it constricts the visual zone of the cornea 

 and diminishes the astigmatic effects of its asymmetrical curvature. 

 But the main function of the iris is to regulate the amount of light 

 that enters the eye and obviate a glare, and thus enable the eye to 

 function as a perfect camera obscura. As the term refractive 

 accommodation is applied to the gradual change in the refractive 



