238 



proceedings: philosophical society 



centimeter (frequency differences) to be constant to 1 part in several 

 millions. 



An extensive application of these standardized wave-lengths has 

 been made to an investigation of spectra in the red and adjacent infra- 

 red regions. The arc spectra of 20 of the chemical elements have been 

 photographed from 6000 A to 9000 A and beyond. The photographs 

 were made in the first order spectrum of a concave grating of 640 cm. 

 radius. Ordinary photographic plates stained in dicyanin solution 

 were used and found to be quite sensitive to the long waves. Wave- 

 length measurements on the international scale have been made in 

 these spectra and most of the frequency differences in the spectra of 

 the alkali metals are shown to be constant to 1 part in several hundred 

 thousand of the wave number. 



Mr. F. E. Fowle presented an illustrated paper on Spectroscopic 

 field light. When a spectrum is formed the energy observed at any 

 wave-length is not simply what is proper to that wave-length but it 

 is diminished by a portion scattered into other parts of the spectrum 



TABLE 1 

 Spectroscopic Field Light 



1. Wave-length in fx 



2. Deviation in minutes. 



3. Quartz transmits 



4. Ditto corrected 



5. Total field light 



6. Black-body radiation.. 



7. Nernst-lamp radiation 



17.5 

 100 



8 

 10 

 11 

 20 



2 



and increased by portions scattered in from other regions. In measure- 

 ments in long-wave spectra this scattered radiation or field light may 

 become very important and troublesome. Rock-salt prismatic energy 

 curves were made of the radiation from a Nernst lamp with a half- 

 centimeter thick quartz plate inserted between the energy source and 

 the silt of the spectroscope. The quartz is nearly transparent for 

 light waves less than 4/z in length, and opaque for those longer. The 

 resulting energy curves consisted of two parts of quite different signi- 

 ficance: 1st, a nearly symmetrical sharp maximum, the energy curve 

 of the source to 4^; 2nd, beyond this the energy curve of the light 

 scattered from this region into that for which quartz is opaque. This 

 central maximum with a wing-like appendage on each side was assumed 

 to represent closely the energy curve of a monochromatic line with the 

 radiation scattered to each side in the spectrum. This curve was 

 used to compute the energy scattered into and away from each region 

 of the spectrum. A summary of the results is given in Table 1 in 

 which the captions in the first column have the following significance: 

 (1) Wave-lengths in millionths of a meter m; (2) Deviation differ- 

 ences in minutes of arc, 15° rock-salt spectrum, zero deviation at 1.8m; 



