LABORATORY MANUAL OF PHYSIOLOGY. 



1. Refraction. A ray of light passing through one transparent 

 medium into another of different density is bent or refracted, 

 unless the ray falls perpendicular to the surface of the denser 

 medium. The ray is spoken of as incident before entering the 

 second medium, and emergent after leaving it. 



Upon entering a denser medium, the ray is refracted toward the 

 perpendicular and from the perpendicular upon entering a rarer 

 one. Reflection accompanies refraction, the ray dividing at the 

 point of incidence. 



THE INDEX OF REFRACTION is the relative resistance of a sub- 

 stance to the passage of light. Air is taken as a standard and is 

 called i. The index of refraction of water is 1.3, of glass, 1.5. 

 The diamond has the highest refractive index, which is 2.4. 



LENSES. A lens is a transparent substance, usually glass, bound- 

 ed by two curved surfaces, or by one plane and one curved surface. 



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FIG. 40. , Object ; /', image ; n, nodal point ; L, lens ; F, F/, and/,/, conjugate foci. 



It may be regarded as a series of prisms. In a convex lens the 

 bases are directed toward the centre, and in a concave lens the 

 bases are directed away from the centre. Rays of light passing 

 through a convex lens are made to converge. Those passing 

 through a concave lens are made to diverge. 



The point to which rays converge, after passing through a con- 

 vex lens, is its focus. The principal focus of a convex lens is its 

 focus for parallel rays. 



When rays of light diverge from any point nearer than infinity, 

 they are brought to a focus at a point beyond the principal focus. 



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