196 Mr. S. P. Langley's Experimental Determination of 



considerations of a very practical kind to take as our normal 

 or standard scale that of the wave-length itself. 



Since we have this normal spectrum actually before us, 

 through the concave gratings constructed by Professor Row- 

 land, it may seem as though we might dispense with the 

 prism ; but this is not as yet possible for the lower part of 

 the spectrum, where overlapping spectra and feeble heat 

 make the use of the grating too difficult. If we could use 

 the solar energy here, not in the form of heat, but of 

 chemical action, as in photography, a great advance might 

 be made ; but photography has not yet advanced beyond the 

 upper part of the great infra-red region. At present we 

 have only heat, and the thermopile or the bolometer ; which 

 latter, though less sensitive than the camera, can be made, as 

 I shall show, to determine experimentally within known 

 limits of error the actual wave-lengths corresponding to 

 given indices of refraction, and hence to afford here valid 

 experimental data for passing from the prismatic spectrum to 

 the normal one. The reason why this so desirable infor- 

 mation has never been obtained before is twofold : — (1) 

 While the measurement in question can best be made by 

 means of a prism and grating conjointly, the heat, which in 

 the lower prismatic spectrum is very faint, becomes almost a 

 vanishing quantity when it has passed the grating also, 

 where the heat is on the average less than one tenth that 

 from the prism. We must use too, if possible, a narrow 

 aperture to register this heat; for a broad one might (on 

 account of the compression of the infra-red by the prism) 

 cover the whole field in which its work should be to dis- 

 criminate. (2) We must have not only an instrument more 

 sensitive than the common thermopile, but we must devise 

 some way of fixing with approximate precision the point at 

 which we are measuring, when that point is actually invisible. 



The apparatus I have devised for this double purpose has 

 done its work with a degree of accuracy which, if it may be 

 called considerable as compared with what we have been 

 used to in heat-measurements, is yet necessarily inferior to 

 that obtained by the eye, and less than we may hope for at 

 some future time from photography. Nevertheless it has, I 

 believe, given experimental data very far outside the visible 

 spectrum, by which we may either construct an empirical 

 formula and supply its proper constants so that it will be 

 trustworthy within extended limits, or test the exactness of 

 such formulae as Cauchy's, Pedtenbacher's, &c, which, while 

 professing a theoretical basis, only agree in their results 

 within the limits of the visible spectrum (from which they 



