Oct. 12, 1882] 



NATURE 



587 



lished by Capt. Abney, but except some very slight indications 

 by Lamansky, Desains, and Mouton, no further guide. 



Deviations being proportionate to abscissae, and measured 

 solar energies to ordinates, we have here (1) the distribution of 

 energy in the prismatic and (2) its distribution in the normal 

 spectrum. The total energy is in each case proportionate to the 

 area of the curve, (the two very dissimilar curves inclosing the 

 same area), and on each, if the total energy be roughly divided 

 into four parts, one of these will correspond to the visible, and 

 three to the invisible or ultra-red part. The total energy at the 

 ultra violet end, is so small then as to be here altogether 

 negligable. 



We observe that (owing to the distortion introduced by the 

 prism) the maximum ordinate representing the heat in the pris- 

 matic spectrum is, as observed by Tyndall, below the red, while 

 upon the normal scale this maximum ordinate is found in the 

 orange. 



I would next ask your attention to the fact that in either 

 spectrum, below \= 12,000 are most extraordinary depressions 

 and interruptions of the energy, to which, as will be seen, the 

 visible spectrum offers no parallel. As to the agent producing 

 these great gaps, which so strikingly interrupt the continuity of 

 the curve, and as you see, in one place, cut it completely in two, 



I have as yet obtained no conclusive evidence. Knowing the 

 great absorption of water vapor in this lowest region, as we 

 already do, from the observations of Tyndall, it would, a priori, 

 seem not unreasonable to look to it as the cause. On the other 

 hand, when I have continued observations from noon to sunset, 

 making successive measures of each ordinate, as the sinking sun 

 sent its rays through greater depths of absorbirg atmosphere ; I 

 have not found these gaps increasing, as much as they apparently 

 should, if due to a terrestrial cause, and so far as this evidence 

 goes, they might be rather thought to be solar. But my own 

 means of investigation are not so well adapted to decide this im- 

 portant point, as those of photography, to which we may yet 

 be indebted for our final conclusion. 



I am led from a study of Capt. Abney's photographs of the 

 region between A.= 8,coo and \= 12,000, to think that these 

 gaps are produced by the aggregation of finer lines, which can 

 best be discriminated by the camera, an instrument, which 

 where it can be used at all, is far more sensitive than the bolo- 

 meter ; while the latter, I think, has on the other hand some 

 advantage in affording direct and trustworthy measures of the 

 amount of energy inhering in each ray. 



- One reason why the extent of this great region has been so 

 singularly underestimated, is the deceptively small space into 



Fig. 1.— Prismatic Sp 



which it appears to be compressed by the distortion of the 

 prism. To discriminate between these crowded rays I have 

 been driven to the invention of a special instrument. The 

 bolometer, which I have here, is an instrument depending upon 

 principles which I need not explain at length, since all present 

 may be presumed to be familiar « ith the success w hich has before 

 attended their application in another field, in the hands of the 

 President of this Association. 



I may remark, however, that this special construction has 

 involved very considerable difficulties and long labcur. For the 

 instrument here shown, platinum has been rolled by Messrs. 

 Tiffany, of New York, into sheets, which as determined by the 

 kindness of Professor Rood, reach the surprising tenuity of less 

 than y^cTT of an English inch (I have al o iron rolled to TT jTnr 

 inch), and from this platinum a strip is cut xfj of an inch wide. 

 This minute strip, forming one arm of a Wheatstone's bridge, 

 and thus perfectly shielded from air currents, is accurately 

 centred by means of a compound microscope, in this truly turned 

 cylinder, and the cylinder itself is exactly directed by the arms 

 of this Y. 



The attached galvanometer responds readily to changes of 

 temperature, of much less than i„fo,y Fah. Since it is one 

 and the same solar energy, whose manifestations we call " light " 

 or "heat," according to the medium which interprets them, what is 



" light " to the eye is " heat " to the bolometer, and what is seen 

 as a dark line by the eye is felt as a cold line by the sentient instru- 

 ment. Accordingly if lines analogous to the dark " Frauenhofer 

 lints" exist in this invisible region they will appear (if I may so 

 j speak), to the bolometer as cold bands, and this hair-like strip 

 of platina is moved along in the invisible part of the spectrum, 

 till the galvanometer indicates the all but infinitesimal change of 

 temperature caused by its contact with such a "cold band." 

 The whole work, it will be seen, is necessarily very slow; it is 

 in fact a long groping in the dark, and it demands extreme 

 patience. A portion of its results are now before you. 



The most tedicus part of the whole process, has been the 

 determination of the w ave-Iengths. It will be remembered that 

 we have (except through the work of Capt. Abney, already cited, 

 and perhaps of M. Mouton), no direct knowledge of the wave- 

 lengths in the infra-red prismatic spectrum, but have hitherto 

 inferred them from formulas like the well-known one ot 

 Cauchy's, all which known to me appear to be here found 

 erroneous by the test of direct experiment ; at least in the case 

 of the prism actually employed. 



I have been greatly aided in this part of the work by the 

 remarkable concave gratings lately constructed by Prof. 

 Rowland of Baltimore, one of which I have the pleasure of 

 showing you. (Instrument exhibited.) 



