686 THE NEW SPECTRUM. 



scale of some kind (whicli he does p.ot furnish), hut tlu'V j)rol>:il)ly indi- 

 cate something, goiny- down to near a w:iv(»-leno-th of 1^. It is obvious 

 that the detail is of th(> very crudest, and yet this drawing- of Lamansky's 

 was remarkahh' as the lii'st drawing of tlie energy spectrum. It at- 

 tracted general attention, and was the immediate cause of the writer's 

 taking up his researches in this direction. 



It seems proper to state here that the true wave-lengtlis were at 

 that tiujc most imperfectly known. l)ut that in 1884. and later in 1885,' 

 they were coniplet(dy determined 1)V the writer as far as the end of 

 what he has called "the new spectrum" at a wave-length of 5.3". 



The upper portion of thi^ i?ifra-red is ({uite accessible to photog- 

 raphy, and the next impoi-tant publication in this direction was that 

 of Captain (now Sir \\'illiam) Abnev." which gave the photographic 

 spectrum down to about l.l". nuich l)eyond which photography has 

 never mapped since. 



From the time of seeing Lamansky's drawing, the writer had grown 

 interested in this work, but found the thermopile, the instrument of 

 his predecessors, and the most delicate then Known to science, insuffi- 

 cient in the feelile heat of the grating spectrum, and about 1880 he 

 had inventeil the bolometer ' and was using it in that year for these 

 researches, 'J'his may perhaps seem the place to speak of this instru- 

 ment, though w ith the later developments which have made it what it 

 is to-day. it has grown to something very diti'erent from what it was 

 then. 



It has, in fact, since found very general acceptance among i)hysicists, 

 especially since it has lately reached a degree of accuracy, as well as of 

 delicacy, which would liii\t' appeared iin])ossible to the inventor him- 

 self in its early days. 



It may be considered in sev'eral relictions, but notably as to three: 

 (1) Its sensitiveness to small amounts of heat; (2) the accuracy of 

 measurement of those small amounts; and (8) the accuracy of its meas- 

 urements of the position of the source of heat. 



As to the first, it is well known that the principle of the instrument 

 depends on the forming of a Wheatstone bridge, by the means of two 

 strips of platinum or other metal, of narrow' Avidth and still more 

 limited thickness, one of which only is exposed to the radiation. In 

 some bolometers in use, for instance, the strip is a tenth of a milli- 

 meter, or one two-hundred-and-tiftieth of an inch in width; and yet it 

 is to be described as only a kind of tape, since its thickness is less than 

 a tenth of this. 



The use of the instrument is then based on the well-know'n fact that 

 the heating of an ordinary metallic conductor increases its resistance, 



^American Journal of Science, March, 1884, and August, 1886. 



^ Philosophical Transactions, vol. clxxi, p. 653, 1880. 



* Actinic balance, American Journal of Science, 3d series, vol. xxi, j). 187, 1881. 



