12 INFRA-RED TRANSMISSION SPECTRA. 



APPARATUS AND METHODS. 



In the present investigation a mirror spectrometer, a rock-salt prism, 

 and a Nichols radiometer were used. With the exception of certain 

 improvements, the details in mounting and adjusting the different parts 

 have been described elsewhere, 1 and it will be sufficient to add that the 

 spectrometer mirrors were 10 cm. in diameter and 50 cm. in focal 

 length, and that the rock-salt prism was an unusually fine one having 

 a refracting angle of 59 59' 36" and faces 9 by 9 cm. area. The spec- 

 trometer slits were 0.3 mm. wide, or about 2' of arc. The time required 

 for the radiometer to reach a maximum deflection was about 20 seconds. 

 The different portions of the spectrum were projected upon the radi- 

 ometer slit by using the Wadsworth 2 mirror-prism method and rotating 

 the prism-table. Although one doubles the error in making spec- 

 trometer settings, the method was the most convenient one in the 

 present work. The "heater" of a no Nernst lamp on an 8o-volt 

 storage-battery circuit was used as a source of energy. The sections 

 were mounted in the usual manner upon a wooden carrier which moved 

 in vertical ways between the source of energy and the collimator slit. 



To protect the prism from moisture a small crystallizing dish of 

 phosphorus pentoxide was placed over it, and when not in use a large 

 glass crystallizing dish was inverted over the prism and drying mate- 

 rial. The edge of the large dish was ground to fit the prism-table and 

 prevented the entrance of moisture. 



The spectrometer was remodeled from a small Fuess pattern having 

 mirrors 4 cm. in diameter. The focal length of the new mirrors was 

 selected as being more advantageous than the 35 cm. or 100 cm. mir- 

 rors, used in previous work, on account of the increased resolving 

 power over the former and the decreased atmospheric absorption which 

 is apparent in the latter. The advantages gained by the use of the 

 larger prism and mirrors is so much more than anticipated that it 

 seems worth while to mention it here. The whole was inclosed in a 

 tin box to protect the prism from air currents and from moisture. 



The radiometer was of the usual form, with an additional improve- 

 ment which consisted in covering it with a piece of heavy brass tubing, 

 within which was a thick layer of hair felt. Although the radiometer 

 stood close to a large outside window and exposed to sunlight, it was 

 not affected by temperature changes. The vanes were of mica, covered 

 with copper oxide, which was applied by means of shellac or a thin 



1 Phys. Rev., vols. 16 and 17, 1903; Investigations of Infra-red Spectra, Wash- 

 ington, 1905. 



2 Wadsworth: Phil. Mag. (5), 38, p. 346, 1894. 



