LABORATORY MANUAL OF PHYSIOLOGY. 



separated by a spherical surface. In such an apparatus, the 

 optical properties depend upon the curvature of the surface and 

 the refractive power of the media. Such an apparatus is shown 

 in the accompanying figure (Fig. 42). The line pr represents a 

 curved surface separating media of different refractive power, the 

 lens being on the left The line oa, falling perpendicularly upon the 

 surface at 3, passes through the centre of the sphere, 6. The line 

 o-a is the optical axis. All the lines that cut the surface normally, 

 such as o-dj c-y, and u-i, undergo no refraction and, continuing in 

 straight lines, cross at 6, which is the nodal point. All of the rays 

 are refracted. All rays, parallel to the optical axis, passing through 

 the lens will be bent so as to meet at m, which is the posterior 

 principal focus. The anterior principal focus is at b, in the first 

 medium and in front of the lens. Rays of light, such as b-i-t, 

 passing from it, are so refracted that they become parallel to the 

 optic axis. The principal point is the point where the optic axis 

 cuts the surface. The posterior, anterior, nodal, and principal 

 points are the cardinal points of an optical system. 



THE EYE AS AN OPTICAL INSTRUMENT. Having reviewed the 

 general optical principles concerning the refraction of light and 

 the formation of images by convex lenses, we now come to the eye 

 as an optical instrument. Rays of light, as they enter the eye 

 encounter not one refracting medium as in the simple dioptric 

 system, but five, namely: 



Tears, 



Cornea, 



Aqueous humor, 



Lens, 



Vitreous humor. 



The indices of refraction of these various media are such that 

 parallel rays of light, entering a normal eye, are brought to a focus 

 upon the retina. For the sake of simplicity, they may be looked 

 upon as equal to a convex lens of about twenty-three millimetres 

 focus. However, a ray of light falling upon the cornea does not 

 follow the same simple direction it would, were it to pass through a 



