REVERSED AND NON-REVERSED SPECTRA. 



53 



the dispersive power dO/d\ being computed (approximately) from Cauchy's 

 equation, so that 



H = A+B/\ 2 = sin O+5)/2/sin <p/2 

 nearly, and therefore 



d8/d\ = 4B sin ^/2/X 3 cos (<p+8)/2 



(p being the prism angle (60) and 5 the angle of minimum deviation. The 

 constant B was put 4.6Xio~ n . 



TABLE 10. Ranges of displacement, e, y, for different dispersions. Method of figure 14. 



In the remaining series dQ/d\ = i/D cos 6, the usual expression for the grat- 

 ing, 6 being the angle of diffraction and D the grating space. The dispersive 

 power thus increases from about 800 to 17,000, over 20 times. Throughout 

 this whole enormous range good fringes were obtained. 



The values e show the displacement of the opaque mirror M during the 



