162 



Capt. Abney and Col. Festing. 

 Fro. 2. 



[June 19, 



Note . The numbers attached to the unbroken curves indicate the turns of the 

 screw of the pile which define the parts of the spectrum to which the curves 

 respectively refer. The wave numbers of these can be found by a reference to 

 Table X. 



We now come to the method we adopted in measuring the radiation 

 at different parts of the spectrum.* A collimator with a lens of the 

 same glass as the prism was used to render the rays from the 

 incandescence lamps parallel before falling on the prism. Another 

 lens of the same material, fitted on to a camera with horizontal 

 swing back, received the spectrum which was focussed on the screen 

 of the latter. A linear and very delicate thermopile mounted on a 

 stand with a screen was employed ; the accompanying figure will give 

 an idea of the instrument (fig. 3). S is a polished silver slit placed in 

 front of the linear couples, which could be closed at will by a screw D. 

 E is a brass mount for a plate of glass or rock-salt, a similar one being 



* This description was added after the paper was vead, as we were advised that 

 the subject-matter would thereby be rendered more clear. (July 14.) 



