of Infra-Red Rays in Rock Salt and Sylvine. 37 



sin 



<£ + a 



2 



sin 



2 



to the examined portion of the infra-red spectrum, is not, 

 strictly speaking, permissible, as this formula presupposes 

 minimum deviation. Through calculation, however, it appears, 

 that in our case the error is sufficiently small to be neglected, 

 this being due to the acute angle of the prisms used. 



This arrangement, consisting of the slit s , the concave mir- 

 ror e , and the prism p, made it possible for us to throw on the 

 slit of the spectrometer s 2 e 1 e 2 s a , rays of known minimum devia- 

 tion, and, as the angle of the prism had been previously deter- 

 mined, of given refractive index. The corresponding wave 

 lengths were determined by means of the diffraction grating g. 

 The properties and capabilities of this grating have been 

 minutely enough described in the above mentioned communica- 

 tion* to permit us to omit further description. For the measure- 

 ment of the wave length, both diffraction images of the first 

 order were always located and the distribution of energy in both 

 carefully determined in this way. We rotated the movable 

 arm of the spectrometer a minute of arc at a time and each 

 time carefully measured and noted the throw of the radiometer 

 for the point in question. In figure 3 such a series for rock 

 salt at the wave length X — 13'91/u, is represented graphically. 

 The readings on the divided circle of the spectrometer s 1 e l e i s i 

 corresponding to the positions of the movable arm are plotted 

 as abscissas, the corresponding throws of the radiometer as 

 ordi nates. It is noticeable that the diffraction images differ to 

 a marked degree in distribution of energy. The right image 

 is considerably narrower and higher than the left. This is not, 

 as might be imagined, an accidental peculiarity of this series, 

 due to inequalities in the intensity of the light source, but on 

 the contrary is common to all the series observed by us. It is 

 explained by the fact that the rays falling on the slit s 1 are not 

 perfectly homogeneous, but are rather of different wave 

 lengths ; and, in fact, the distribution of the rays on the slit is 

 such that (looking at the slit s 1 from the mirror e ) those of 

 shorter wave length fall on the right edge of the slit, those of 

 greater wave length on the left. The same distribution of 

 energy is apparent in the central image of fig. 3, only here 

 right becomes left, and left right. Both the diffraction images 

 appear widened with respect to the central image on account 

 of the dispersion of the grating ; however, the distribution of 

 energy, already present in the central image, works on the two 



* Nichols and Rubens, TVied. Ann., Ix, p. 425, 1897. 



