18 Prof. A. Gray and Dr. W. Stewart. Radiation of [Apr. 27, 



lin>sC D F G Then from Cauchy's equation were calculated the 

 XL 2' ^responding to the wave-lengths 4000, 4500 5000, 

 5500 and so on up to 8000 tenth-metres. From these values a 

 curve was plotted which gave p for any wave-length. In a similar 

 manner a curve was plotted which gave - Xdp/dX for any value of 

 A Finally we calculated a sufficient number of values of the multi 

 plying factor A*/ (p - 1 - MP /d\) 6, and plotted a curve giving 

 value of this factor for any wave-length. This curve is shown in fig. 1. 



FIG. 1. 



1-0 1 



Y -oe 



& 



-O5 



04 



03 



40OO 45OO 5OOO 55OO 6OOO 6SOO 7OOO 75CO 8OOO 85OO 

 X in tenth-metres 



Arrangement of Apparatus. The source of light was an exhausted 

 tube, containing the vapour the spectrum of which was under examina- 

 tion, provided as usual with terminals and joined up in the secondary 

 circuit of an induction coil ; the capillary part of the tube was placed 

 between the poles of the electromagnet. A convergent lens was 

 placed so as to bring the light from the tube to a focus on the 

 collimator slit of the auxiliary spectroscope, which effected a 

 preliminary partial resolution of the light before it fell on the slit 

 of the collimator used directly with the echelon. This auxiliary 

 spectroscope was of the constant deviation kind, and had the collimator 

 and telescope fixed at right angles to one another. The prism was 

 a species of internal reflection prism, made up of two 30 prisms and 

 one right-angled prism, cemented together as shown in fig. 2. The 

 light entered at the face a, underwent internal reflection at the 

 hypotenusal face 5, and emerged at the face c. With this arrange- 

 ment the part of the spectrum in the middle of the field of view was 



