ROTATORY POLARIZATION. 247 



section. The planes of polarization of these two portions are 

 however different, and form an angle nearly equal to double 

 the angle which that plane contains with the longer diagonal 

 of the base of the prism. Hence the intensities of the two 

 halves will be unequal, unless when the plane of the section 

 coincides with the plane of primitive polarization. The ex- 

 periment then consists in equalizing the intensities by the 

 rotation of the prism ; and as the eye is capable of detecting 

 with much accuracy differences of intensities of two lights 

 observed simultaneously, the position of the plane of primi- 

 tive polarization will be determined with great exactness. 

 With a well-constructed prism of this kind, the error of ob- 

 servation need not exceed 1', that of the ordinary method 

 being about 30 '. 



Mr. Jellett has contrived a saccharometer in which a 

 prism, such as has been above described, is used to determine 

 the amount of rotation of the plane of polarization produced 

 by a saccharine solution, and therefore the strength of the 

 solution itself. He estimates the probable error of the result 

 of a single observation, with such an instrument, to be only 

 0-02 of a grain in a cubic inch of the solution. 





THE END. 



