ON RECENT RESEARCHES IN THE INFRA-RED SPECTRUM. 473 



Immediately below this curve is the more familiar linear representa- 

 tion of the same subject. Now this linear representation, it is most 

 important to observe, has been obtained^ not by drawing;, but by the 

 subsequent application to the curve of an automatic process, by means of 

 which its indications are reproduced by photogravure, as separate lines ; 

 while, by the same automatic process, the most complex spectral curves 

 can be rendered into their linear equivalents. 



I have no space to enter here on a description of this process, further 

 than to say it is effected by means of a systematically distorted image of 

 the curve, obtained by a special combination of spherical and cylindric 

 lenses. You will see, on minute inspection, that the inflections of th& 

 galvanometer curve have been slightly ' loaded,' to produce a more effec- 

 tive contrast of light and dark. Except for this, which can in no way 

 affect the position of a line, but only its intensity, the whole process is as 

 absolutely automatic as any photograph of the visible spectrum. 



This thermograph of the 'D' lines has been chosen to indicate the 

 grasp of this new thermometric method, by applying it to ihe test of an 

 object in the visible spectrum with which every physicist is doubtless 

 familiar. He may then be invited to recall that the distance between the 

 ' D's ' in a rock-salt sixty-degree prism is about eleven seconds of arc, and 

 to observe that two lines, about half this distance apart, are here shown 

 as sharply divided by this thermal method as, for instance, are the com- 

 ponents of the double star a Geminorum by a 3-inch achromatic. 

 Obviously, then, our method could indicate the existence of two lines, 

 little, if any, more than one-quarter the distance between the ' D's.' 

 Lines 3" or less apart can, then, evidently, be indicated l;y tliis method, 

 even in its present state of development. 



And now, returning to what has been said about the evidence obtain- 

 able as to the perfect coincidence of these inflections in different energy 

 curves obtained at different times, and to the consequent evidence that 

 each inflection so given is real, and not the product of an accidental 

 variation in the curve, we may conceive that from any number of such 

 independent curves any number of such linear representations of the 

 spectra have been obtained ; for example, that ten such linear representa- 

 tions of the whole spectrum as are here given of the ' D ' lines only, have 

 been so found from ten complete energy curves taken on as many diffei'enb 

 days. From these ten linear representations, by the well-known pro- 

 cesses of composite photography, one final photograph of the spectrum is 

 formed, and on this it is evident we may expect to find only what is 

 permanent and not what is accidental, granting that a rare accident may 

 have introduced an occasional abnormal deflection. 



Now considering that the part of the infra-red solar spectrum of rock- 

 salt under review extends through nearly two degrees, or 7,200 seconds, 

 and that we have just seen by the illustration of the ' D ' lines (fig. 2) 

 that lines 3" apart can be thus indicated, we may see for ourselves that at 

 any rate over 2,000 lines, if they exist, can be mapped. But these lines 

 do exist, the whole of this new region being apparently as intimately 

 filled by them as the visible spectrum by the Fraunhofer lines. In f urthey 

 evidence of this I now show a portion of the lower spectrum (fig. 3, 

 Plate III.) ' in the comparatively unknown part extending from /\=l^-4 to 

 A=2'"2, including the great band Q, shown as a single inflection in my first 



' This figure, exhibited here only in illustration, is not to be treated as a criterion 

 of the final rusults to be attained bj' the composite process. 



