﻿86 S. P. Langley — Unrecognized Wave-lengths. 



from the electric arc, is rendered evident when we state that 

 for these extreme wave-lengths, the arc radiation is compara- 

 tively so small, that the heat from the hottest part of the 

 dazzlingly bright carbon does not very greatly exceed that from 

 a piece of melting ice. If, then, we are to distinguish here be- 

 tween the radiation which passes through the open slit from 

 the incandescent carbon and that which comes from the ad- 

 jacent edges of the slit, which inevitably mingles more or less 

 with the former, the difference between the two temperatures 

 must be made as great as possible. 



Following now on Plate III the course of the ray from 

 this electric arc, we observe that it falls on the grating 

 G (to be presently described), which spreads it out into 

 not one but many superposed spectra, distributed along 

 the circumference of a circle whose center is at equidistant 

 from Gr, Si, and S 2 . For clearer illustration, let us sup- 

 pose ourselves about to measure the heat in some ray of 

 the visible spectrum (such as that near D 2 , whose wave- 

 length is nearly (f'6) and that the line S 2 S 2 is a scale of equal 

 parts. In this case, the beam A will be moved so that while 

 the grating remains at the intersection of N S and Gr A and 

 normal to the latter, the slit S 2 will be brought close to S x in the 

 position O^'ft on our scale of equal parts, whose zero is at S x . 

 Here, (if we suppose sunlight to be employed,) we shall see a 

 brilliant spectrum filled with Fraunhofer lines crossing the front 

 of the plate of the slit S 2 . Beyond this the second, third and 

 other higher orders of spectra are distributed on the circum- 

 ference of the circle in which S 2 always lies. Were it our 

 only object to discriminate the heat in this particular visible 

 ray, we should not in this case need the slit S 2 or the prismatic 

 train, but could place the bolometer directly at S 2 . Since we 

 make the ordinary use of the slit S 2 however, we suppose our- 

 selves to be determining the prismatic dispersion, for a given 

 wave-length, that is, it forms with the prismatic train an 

 approximately homogeneous spectral image at B, which can be 

 viewed, or measured with the bolometer, giving the value of n 

 for a known value of X. For the mere purpose of measuring 

 the heat in the ray, or determining its wave-length here in the 

 visible part, where there is but a single sensible heat-spectrum, 

 we do not need slit S 2 at all, while the refractive index for a 

 glass prism could be as easily determined as that for rock-salt. 

 Besides this, we should find here a relatively abundant heat 

 and could use so narrow a bolometer as to fix the position by 

 it alone quite accurately. Yery different, however, are all 

 the conditions if we wish to measure, for example, a wave- 

 length corresponding to 3^ or 4^ in the invisible spectrum and 



