ON LIGHT. 247 



always have their sines in a fixed proportion, the greater 

 may increase up to a right angle, but the less cannot ; 

 since the contrary would require the sine of the greater 

 to exceed the radius of the circle. 



(31.) Within this limit, when the angle of incidence 

 is such as to admit of the transmission of the ray, the 

 reflexion is less than total. The incident beam is sub- 

 divided ; a part only is transmitted, the rest undergoes 

 reflexion. The total amount of incident light is divided 

 between them, but very unequally, and the more so the 

 less the difference between the refractive indices of the 

 media ; or, in optical language, between their " refrac- 

 tive densities." Thus, when light passes at a perpen- 

 dicular incidence out of air into water, only 2 per cent, 

 of the whole incident beam is reflected; when into plate- 

 glass, about 4 per cent, but when out of water into such 

 glass, the amount of reflected light is less than \ per 

 cent. At oblique incidences, the reflexion is more 

 copious, increasing in intensity as the obliquity in- 

 creases, until the incident light but just grazes the 

 surface. 



(32.) The laws of reflexion and refraction being known, 

 it is the part of geometry to follow them out in the 

 several cases where light is incident on plane, spherical, 

 or any other curved surfaces, reflecting or refracting, 

 and thus to deduce the various theorems and proposi- 

 tions which the practical optician has need of for the 

 construction of his mirrors, lenses, prisms, telescopes, 

 and microscopes. All these, as beside our present pur- 

 pose, we pretermit, confining ourselves entirely to the 



