S. P. Langley — Observations on Invisible Heat-Spectra. 9 



of the sources of emission, but chiefly with the secondary effect 

 of the progressive movement of the maximum which is clearly 

 shown. 



Date of observation. 



Approximate temper- 



Approximate tempera- 



Deviation of maximum 





ature of source. 



ture of excess. 



ordinate in heat curve. 



1885. 









October 7, 



815° C. 



803° 0. 



39°-08' 



" 3, 



525 



505 



39-03 



7, 



330 



318 



39-01 



" 3, 



330 



310 



39-00 



September 26, 



300 



275 



3842 



March 20, 



17S 



185 



38 35 



August 19, 



179 



152 



38-35 



March 20, 



119 



126 



38-25 



October 7, 



100 



88 



38-22 



" 3, 



99 



79 



38-27 



March 21, 



40 



46 



3800 



" 24 



— 02 



— 18 



37-40* 



It is to be observed of each of the curves in Plate 3, that 

 though nearly all the area is seen, yet that, owing to the exten- 

 sion of the heat curve toward the right, the length shown is 

 limited here by the size of the plate, whereas the extremity 

 measured in each curve (except of course the solar one) does, 

 in fact, correspond to an index of less than 1*45. We give 

 above a table showing the dates of observation, the approxi- 

 mate temperatures of excess and the approximate deviations of 

 the ordinate corresponding to the point of maximum heat in 

 the (rock-salt) prismatic spectrum. We should observe that 

 the higher temperatures are here only determined with an 

 approximation sufficient to make it certain that there is a pro- 

 gression in the direction of the shorter wave-lengths of the 

 position of the maximum heat-ordinate corresponding to the 

 temperature, as the latter rises. These results, both in the 

 table and as represented in the plates, are given as preliminary, 

 not as final, for we hope soon to be able to offer other and more 

 exact ones, deduced from the heat spectra of bodies at all 

 temperatures between that of melting platinum and melting 

 ice. We are entitled, however, even at present, to draw the 

 following conclusions, which are of special interest in connec- 

 tion with the spectra of dark bodies, of which almost nothing 

 has been hitherto known. 



(1) The heat represented by the areas of these curves is 

 almost altogether of a character not observed in that of the sun, 

 these wave-lengths, in general, not being transmissible by 

 glass, which is comparatively permeable to the lowest solar 



* The position of the maximum in this latter case depends upon a single obser- 

 vation of some delicacy and is liable to subsequent correction. 



