478 S. P. Langley — Optical Properties of Rock- salt. 



have been made for us by Messrs. Alvan Clark & Sons, of 

 Cambridge, and Mr. J. A. Brashear, of Pittsburgh, who has 

 worked that here described and which shows the nickel line 

 between the D's. 



For the reduction and arrangement of these observations a 

 more complete knowledge of the properties of rock-salt than 

 had hitherto been obtained was essential, particularly as regards 

 its indices of refraction for raj's of determinate wave-length, 

 and its diathermancy for dark heat rays of different degrees of 

 refrangibility. 



Advantage has been taken of the use of the train -above 

 mentioned to determine, not only the indices, which will pres- 

 entty be given, but also the the apparent transmission of rock- 

 salt plates in different parts of the spectrum, but the latter part 

 of the work is not represented "here. We shall only observe that 

 we have had occasion to form "heat "-spectra from radiating 

 sources below the temperature of melting ice, and that while 

 most ofthe rays, even from these sources, passed freely through 

 the prism ; with the smallest deviations corresponding to wave 

 lengths, probably exceeding 100,000 of Angstrom's scale, a 

 slight absorption began to be noticed. We hope to shortly give 

 more full determinations of this, in connection with a statement 

 of the deviations and wave-lengths of heat from sources at all 

 degrees between the temperature of melting platinum and that 

 of melting ice, with which it will appear in a more proper con- 

 nection. 



Although in working with such heat radiations even as those 

 forming a part of the solar infra-red spectrum, more error is to 

 be expected than in the optical observations, these errors are, 

 with our present apparatus, of an order not wholl}?" incompar- 

 able with the optical ones. 



It is however to be understood that the best heat spectrum 

 work can only be accomplished with brightly polished rock-salt. 

 The surfaces of the rock-salt prism and lenses undergo a deteri- 

 oration when exposed to the air, which is more or less rapid ac- 

 cording to the greater or smaller relative humidity of the atmos- 

 phere at the time. In ordinary dry weather they may be used 

 several times before they become spoiled, while in damp or rainy 

 weather, three or four seconds is a sufficiently long time to cover 

 them with condensed moisture, and work under these circum- 

 stances is of course impossible. After the surfaces have in this 

 way become unfit for use they are repolished, and the refracting 

 angle of the prism is thereby unavoidably altered. The change 

 is usually small, generally not exceeding ]/, so that for most of 

 our heat measures it may be neglected altogether. The changes 

 have however tended on the whole to reduce the refracting 

 angle, so that it is now about 4' smaller than when the prism 

 was first used. 



