Prof. S. P. La.ngley on the Xew Spectrum. 127 



existed, it was apparent that fifty years would be required to 

 denote them. 



The writer then devised a second apparatus to be used in 

 connexion with the bolometer. This apparatus was simple 

 in theory, though it has taken a dozen years to make it work 

 well in practice, but it is working at last, and with this the 

 maps in this volume o£ the 'Annals'' and that before us 

 have been chiefly made. It is almost entirely automatic, 

 and, as it is now used, a thousand inflections can be delineated 

 in a single hour, much better than this could have been done 

 in the half -century of work just referred to. 



Briefly, the method is this : A great rock-salt prism (for 

 a glass one would not transmit these lower rays nor could 

 they easily bd detected in the overlapping spectra of the 

 grating) is obtained of such purity and accuracy of figure, 

 and so well sheltered from moisture, that its clearness and 

 its indications compare favourably, even in the visible 

 spectrum, with those of the most perfect prism of glass, with 

 the additional advantage that it is permeable to the extreme 

 infra-red rays in question. This prism rests on a large 

 azimuth circle turned by clockwork of the extremest precision 1 

 which causes the spectrum to move slowly along, and in one 

 minute of time, for example, to move exactly one minute of 

 arc of its length before the strip of the bolometer, bringing 

 this successively in contact with one invisible line and 

 another. Since what is blackness to the eye is cold to the 

 bolometer, the contact of the black lines chills the strip and 

 increases the electric current. The bolometer is connected 

 by a cable with the galvanometer, whose consequent swing 

 to the right or the left is photographically registered on a 

 plate which the same clockwork causes to move synchronously 

 and uniformly up or down by exactly one centimetre of 

 space for the corresponding minute. By this means the 

 energy-curve of an invisible region, which directly is wholly 

 inaccessible to photography, is photographed upon the plate*. 



Let it be noted that whatever the relation of the movemei.t 

 of the spectrum to that of the plate is (and different ones 

 might be adopted), it is absolutely synchronous — at least to 

 such a degree that an error in the position of one of these 

 invisible lines can be determined, as litis been stated, with 

 the order of precision of the astronomical measurement of 

 visible things. 



The results were before them in the energy-curves and 

 the linear infra-red spectrum, containing over seven hundred 

 invisible lines. This is more than the number of visible ones 

 in Kirchhoff & Bnnseifs charts. The position of each line is 



