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LXXXIV. Vacuum Spectrometer. 

 By Professor Augustus Trowbridge *. 



[Plate XVI.] 



IT^OR the purpose o£ spectroscopic investigation in the 

 extreme ultra-violet region of the spectrum it has been 

 found necessary, on account of atmospheric absorption, to 

 employ some form of vacuum spectrometer, either of the 

 mirror type or of that where collimation is effected by lenses 

 of some transparent material. 



The instruments employed in the well-known work of 

 Schumann and of Lyman could hardly be improved on were 

 one to design an instrument for research in the ultra-violet 

 region exclusively, and I therefore only venture to describe 

 an instrument which I have recently had constructed because 

 of its wider range of usefulness. 



In research work in the infra-red region of the spectrum, 

 it is customary to use a mirror spectrometer of the fixed arm 

 type with a rock-salt prism and AVadsworth mirror. The 

 energy measuring instruments most commonly employed use 

 the bolometer or the thermopile. It is generally necessary 

 to shield either of these very carefully by means of screens 

 from irregular changes of temperature, and it is not un- 

 common in the use of the bolometer to mount it in an air- 

 tight case with a transparent window. Some observers have 

 worked with the bolometer in vacuo in order better to secure 

 constant temperature conditions. 



If rock-salt be employed as the prism substance it is 

 necessary to protect it against moisture, and this requires 

 that the prism be enclosed in a case with the necessary 

 openings for the passage of the light and suitable arrange- 

 ments for preserving a moisture-free atmosphere within it. 



From the above it is evident that a vacuum spectrometer 

 would be advantageous, though not of course absolutely 

 essential, in securing good working conditions in infra-red 

 investigations as w T ell as in work in the ultra-violet. 



The instrument described in the present paper is designed 

 to be used w r ith either a prism or a grating, and attachments 

 are provided which allow the use of either bolometer or 

 thermopile for w r ork in the infra-red or a photographic plate 

 carrier for work in the ultra-violet. 



Referring to the first of the figures, which are photographic 

 reproductions of the shop drawings, and are one-quarter of 



* Communicated by the Author. 



