VISION. 105 



different regions are, in millimeters, as follows : Tongue I ; 

 finger-tip 2 ; lip 4 ; neck 20 ; back 60. 



Muscular Sensation. 



Muscular exertion is attended with a "sensation of 

 effort," the relation of which to the work done (e.g. the 

 weight lifted), according to Weber's experiments, is such, 

 that no difference less than a fortieth between two weights 

 lifted in succession can be appreciated. The sensation of 

 effort is therefore more delicate than the sensation of 

 pressure. The existence of sensory nerve-endings in the 

 sartorius muscle of the frog has lately been demonstrated ; 

 with this exception, channels of muscular sensation have 

 not hitherto been recognized. 



Vision. 



The Eye as an optical instrument. To understand the 

 paths of luminous rays through the eye to the retina, it is 

 necessary to know the form of its three principal refracting 

 surfaces, and the refractive indices of its transparent 

 media. Of the normal eye the radii of curvature, indices 

 of refraction, and dimensions are approximately as follows : 

 Radius of the corneal surface, 8 millimeters, radius of 

 the anterior surface of lens, 10 millimeters, of posterior 

 surface, 6 millimeters, these surfaces being severally 4 

 millimeters apart in the axis of the eye ; the distance from 

 the posterior surface to the retina is 13 millimeters. The 

 index of refraction of the aqueous or vitreous humour is 

 1-35, that of water being 1*336. The refraction-index of 

 the lens varies from 1-405 at the surface to 1-454 in the 

 centre. 



An eye supposed to be constructed according to these 

 measurements is denoted by the term "schematic eye," 



