346 



BUTTEE. 



and tlie ratio between the angle made by the path of the ray 

 with the perpendicular to the surface in the first medium and 

 the angle made by the path in the second medium with the 

 perpendicular is a constant ; the ratio of the sines of the two 

 angles is known as the index of refraction. As the sine of an 

 angle of 90° is 1, it is seen that the ratio between the sine of the 

 angle at which light is first reflected and 1 is the index of refrac- 



^^ 



Fig. 48. — Oleo-refractoraeter. 



tion ; this angle is termed the angle of total reflection. As it is 

 more convenient to measure this than to measure the two angles, 

 and deduce the ratio of the sines, in practice the angle of total 

 reflection is frequently measured. 



Miiller was the first to apply the determination of the refrac- 

 tive index to the analysis of butter. He allowed the butter to 



