198 



THE ROYAL SOCIETY OF CANADA 



A quartz prism spectrograph and Wratten panchromatic plates 

 were used. Exposures ranged from 2 min., when no filter was used 

 to 2 hours in the case of dense filters, in order to make certain that no 

 radiation of short wave-length could pass without being noticed. 



(3) Apparatus 



The apparatus used is shown in Fig. 1. It is fully described 

 by McLennan and Dearie.^ The radiation from a 400 watt nitrogen 



TO GAL^AAIOMerCR 



Fig. 1 



filled lamp was focussed on the slit Sx by means of a glass lens of 

 28 cms. focal length and 10 cm. diameter. _ From this slit the rays 

 passed to the nickel steel concave mirror Wi, thence through the 

 rock-salt prism p to the plane nickel mirror m-i. From this they 

 were reflected to the concave nickel steel mirror rriz, and by it they 

 were brought to a focus on the linear junctions of the thermopile at 

 /, which was placed immediately behind the slit 52. The prisms and 

 plane mirror were mounted on a table which rotated about a point a. 

 By turning this table through a small angle any desired part of the 

 spectrum could be brought to a focus at 5. The rotation was pro- 

 duced by the motion of a helical drum attached at d, which was 

 calibrated in wave-lengths up to lO^u. An eyepiece e could be 

 attached behind the slit ^2 for the purpose of focussing lines in the 

 visible part of the spectrum, on the thermopile, and of adjusting the 

 prism so that the radiation brought to a focus at ^2 was in agreement 



with the reading on the drum. ^ 



«McLennan and Dearie, Phil. Mag. 30, p. 683, 1915. 



