Radiometric Measurements of Constants of Indicators. 49 



detected by the radiomicrometer. If such proved to be the case, it 

 would be found necessary to introduce color screens to eliminate 

 the light energy from the second-order spectrum. That the first- 

 order spectrum can be regarded as pure is shown by the following 

 considerations : 



Glass cuts off all wave-lengths of light beyond X = 0.35/x. Since the 

 glass in the path of the light is of considerable thickness, the first-order 

 spectrum is not contaminated with any light from the second-order 

 spectrum as far out as X = 0.7 /JL of the first order. A solution of copper 

 sulphate is entirely transparent to all wave-lengths of light shorter 

 than X = 0.6/i. A 10 mm. depth of a saturated solution of copper 

 sulphate shows complete absorption beyond X = 0.6 //. Using the above 

 solution of copper sulphate, radiometric measurements were made 

 from X = 0.4ju to X = 1.3;u. Beyond X = 0.6/x the solution of copper 

 sulphate is entirely opaque, and therefore no evidence of light energy 

 from the second order was obtained as far out as X = 1.3^t. It has 

 already been shown that the first-order spectrum is pure up to X = 0.7/x, 

 and the above shows that the light from the overlapping second-order 

 spectrum does not contain a sufficient amount of energy to be detected 

 by the radiomicrometer as far out in the infra-red as X = 1.2/z of the 

 first-order spectrum. The radiometric measurements of the dissoci- 

 ation constants of the indicators were all made between X = 0.5// and 

 X = 0.6jtt, and in this region the first-order spectrum is absolutely pure. 



The drumhead which rotated the grating through definite angles is a 

 large one, its diameter being 4 inches. It is attached to a very care- 

 fully constructed screw, so designed that each complete revolution 

 changes the wave-length of light falling on the slit by approximately 

 500 A. u. The drum contains 500 divisions, and, all things considered, 

 it is quite probable that the wave-length settings are accurate to within 

 one or two Angstrom units. 



The spectroscope is of the Littrow mounting, the same lens serving 

 both as telescope and collimator. It is provided with two 4-inch lenses 

 made by Brashear. One lens has a focal length of 72 inches and is 

 intended for photographic work. The other, and the one which was 

 used in this work, has a focal length of 30 inches. With this lens, the 

 visible first-order spectrum at slit s 2 has a length of over 6 inches. 

 Provision has also been made for the mounting of a large glass prism 

 on the grating table, and it is purposed to use this prism when an 

 intense first-order spectrum is desired, rather than a spectrum which 

 is widely dispersed. Had we not been successful in constructing a 

 very sensitive radiomicrometer, it would have been necessary to make 

 frequent use of the glass prism. 



The spectroscope was placed in a brass box, blackened on the inside, 

 which prevented stray sources of light from reaching the grating. The 

 slits were of the bilateral type. 



