402 S. P. Langley — Invisible Solar and Lunar Spectrum. 



necessarily equal to the angle of mmimum deviation (R P D, 

 or its equal P Dc). 



Thus, if the pencil of solar radiation reflected from a large 

 siderostat on the north, not shown here, passes from "N toward 

 S, on moving the beam A R, pivoted at p (p being the pro- 

 jection in our drawing of a vertical line passing through the 

 center of the turn-table and the median line of N S and A R) 

 into various positions, N S remaining fixed, the rays, which 

 are refracted by the prism p in the direction of p P, will 

 emerge from P in the direction P D and fall upon the bolom- 

 eter B. A condensing lens (I) forms the solar image on the 

 slit (Sj) of the first spectroscope, whose train (consisting of col- 

 limator, Z t , prism, p, and image-forming lens, Z 3 ) forms a spec- 

 trum on the slit (s 2 ) of the second spectroscope. Here a narrow 

 pencil from the first spectrum comprising only the particular 

 wave-lengths which fall within the width of s a , is admitted, 

 and, by the second train 1^ P L 2 , formed into a horizontal 

 spectrum at and on either side of B. When we move A R 

 this spectrum moves in turn past the vertical linear thread of 

 the bolometer B, which lies in the focal plane of this spectrum, 

 and is immersed in its successive absorption lines as these defile 

 past it. The function of the first spectroscopic train (I, l v p, l 2 ) 

 is solely to sift out the extraneous radiations and to present at 

 the second slit ($ a ) only those which legitimately belong to that 

 part of the spectrum we wish to study. These pure rays pass 

 into the second slit and are analyzed by the second train in the 

 usual way, by the aid of the linear bolometer at B, and of the 

 circle (C) reading to ten seconds of arc. 



The objection to this apparatus is its complexity, which, 

 however, we have been unable to advantageously diminish. 

 "We may, however, satisfy ourselves by visual observation of 

 the Fraunhof er lines seen through the whole compound system ; 

 (entirely of salt) both of the optical perfection of the surfaces 

 of our entire double train, and of the accuracy of its purely 

 automatic action. 



Results of Observation. 



With this and the apparatus already described in previous 

 memoirs we have searched the extreme infra-red solar spectrum 

 at first without definite success, later with results which will 

 be better understood by the accompanying drawings.* Fig. 2b 

 shows the newly investigated invisible solar heat spectrum on 



the normal scale up to an (estimated) wave-length of 18*" . Fig. 



3« is an enlarged view of that portion of it, extending to 5^ 

 and fig. 3b, a photographic interpretation of the last, obtained 

 automatically by a special device ; so as to present somewhat 



* For figs. 2a, 26, 3a, 3b, see Plates IX, X. 



