﻿90 S. P. Langley — Unrecognized Wave-lengths. 



and its positions are read by the divided circle of the spectrom- 

 eter to 10" (ten seconds of arc). It is this simple form of the 

 instrument which has been used in the present investigation, 

 and which is shown in Plate II. The bolometer is shown in 

 position in the middle of the case where its central strip is accu- 

 rately self-centered in the cylinder. For protection from air 

 currents, since the obscure heat studied will be stopped by a 

 glass cover, we must make use of the special device I have de- 

 scribed, in the memoir just referred to, of successive chambers 

 or drums separated by diaphragms with a common central aper- 

 ture. 



With these precautions, and with the special adjuncts before 

 described, a bolometer with a strip -^ mm wide can be set by 

 the invisible heat alone to within 10" of arc, while in the ordi- 

 nary use of the linear thermopile we are liable to errors of a 

 considerable fraction of a degree. Even with a bolometer l mm 

 wide, it will be subsequently seen, we can set to V of arc. 

 This refers only to the precision of pointing attainable ; we will 

 consider the sensitiveness of the instrument later in connection 

 with the galvanometer. 



Galvanometer. 



It must be remembered that while the nominal sensitiveness 

 of a galvanometer can be increased to any extent by increasing 

 the astaticism of the needle (quite as nominal power can be 

 multiplied to any extent on a telescope by altering lenses at 

 the eye-piece), that the real or working capacity depends upon 

 the ability to always obtain a like result under given condi- 

 tions. Accordingly we have continued to devote great pains 

 to extend our original conception, so as to make the galvanom- 

 eter, as well as the bolometer, not merely an indicator of heat, 

 but a real " meter," which shall distinctly answer the question 

 u how much ?" as to almost infinitely minute amounts of energy. 



For the benefit of any physicists who may desire to repeat 

 these experiments, we may observe that we have found the 

 bolometer capable of almost unlimited delicacy of perception of 

 heat, but that our chief trouble has arisen from the difficulty of 

 constructing a galvanometer suitable to develop its full capacity 

 for exact measurement. We have been unable to find among 

 galvanometers ordinarily constructed one capable of indicating 

 with precision changes in the amount of current of much less 

 than T.-g-o^Yoinr °^ an Ampere. It was in the construction of a 

 galvanometer designed to measure the heat in the spectrum of 

 the moon, that we acquired the experience which we have 

 utilized in the present researches. 



A reflecting galvanometer of the form devised by Sir William 

 Thomson has been used for the basis of our construction, and 

 altered as follows. (For several of the changes described I am 



