of Infra- Red Bays in Bock Salt and Sylvine. 39 



Besides this asymmetry of form, there exists a slight but at 

 the same time noticeable asymmetry of position of the two 

 diffraction images with respect to the central image.* The 

 total distance between the two diffraction images of the first 

 order, remains, nevertheless, unchanged. On the other hand, 

 a correction for the position of the maximum of the diffrac- 

 tion spectrum is necessary, for the reason that the rays of 

 shorter wave length, falling on the slit s 19 possess greater energy 

 than those of greater wave length. Because of this, the maxi- 

 mum in both images appears displaced toward the side of the 

 shorter waves. 



4. 



In order to apply this correction, it is necessary to know the 

 distribution of energy in the spectrum thrown from the source 

 of light with the aid of the slit s , the prising and the mirror 

 e . This was therefore carefully measured in each separate 

 case with a special series of observations, and by means of the 

 data so obtained, the angle of diffraction was determined at 

 which the maximum of energy should appear, if the spectrum 

 thrown in the plane of the slit s, possessed the same amount 

 of energy at all points. This determination is easiest made 

 by using a tangential construction given by Paschen,f the 

 details of which are given in figure 4. The curve a represents 

 the intensity curve of a diffraction image ; the curve b gives 

 the distribution of energy in the spectrum thrown in the plane 

 of the slit s x . Let i and i x be two ordinates of the curve b 

 near the maximum of the curve a, and join the points p and q 

 on the curve b corresponding to these ordinates, with a straight 

 line which shall cut the axis of abscissas in a point r, and lay a 



* The cause of the presence of this asymmetry lies in the arrangement of the 

 spectrometer s, e, e 2 s 2 , Rubens and Nichols, Wied. Ann., lx, p. 422, 1897. 

 fF. Paschen, Wied. Ann., li, p. 1, 1894. 



