152 Prof. S. P. Langley on 



Following now on Plate V. 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 centre is at equidistant from Gr, S 1? and 

 S 2 . For clearer illustration $ let us suppose ourselves about to 

 meagre the heat in some ray of the visible spectrum such as 

 that near D 2 (whose wave-length is nearly 0*6), and that the 

 line Si 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 inter- 

 section of N S and G A and normal to the latter, the slit S 2 

 will be brought close to S 2 in the position 0*6 on our scale of 

 equal parts, whose zero is at S x . Here (if we suppose sun- 

 light 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 circumference of the 

 circle in w T hich 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 how T ever, we suppose ourselves to 

 be determining the prismatic dispersion for a given wave- 

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

 mately 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 

 the heat alone quite accurately. Very 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 

 in the new region which we are for the first time exploring. 

 Glass is impermeable to this kind of heat, but with our rock- 

 salt train and with the delicate apparatus previously described, 

 there is little difficulty in discriminating it by the bolometer, 

 where the prism alone is employed, and in mapping the 

 deviation of each spectral ray, as shown in plate vi. (Phil. Mag. 

 May 1886) . But now that we wish to determine wave-lengths, 

 the conditions are altogether different; for now not only does 

 the grating enormously expand this part of the spectrum and 

 diminish the heat correspondingly, just where that heat is itself 



