S. P. Langley — Nevi Spectrum. 411 



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 of 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 cen- 

 tury 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 be 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 favorably, 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 clock- 

 work of the extremest precision, 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 centi- 

 meter 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 movement 

 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 has 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 invis- 

 ible lines. This is more than the number of visible ones in 

 Kirchoff & Bunsen's charts. The position of each line is fixed 

 from a mean of at least six independent determinations with 

 the accuracy stated above. 



The reader will perhaps gather a clearer idea of this action 

 if he imagines the map before him hung up at right angles to 

 its actual position, so that a rise in the energy curve given 



