1883.] On the Solar Spectrum and Solar Temperature. 337 



Scale No. Headings. Scale No. Readings. 



5 75 34 9i 7 



Readings. 



Scale No. 



34 



9* 



2 



19 



10 



18 



101 



28 



11 



38 



11* 



35 



12f 



30 



13 T ^ 



10 



20 



134 



5 



19 



1 



25 



1H 



28 



15 



21 -5 



16 



2 



6f 18 101 22 



7 28 11 25 



7i 38 111 i 6 



71 35 124 



7 



4-5 



8 







1 







Diagram IV shows the continuous curve of Diagram III trans- 

 ferred to the normal scale ; and that in dotted line is Langley's 

 curve ("Phil. Mag.," March, 1883) transferred to our scale, so that 

 the maxima correspond. The chain-dotted curve is curve ~No. VI', 

 Diagram II ; but it must be borne in mind that the vertical scales 

 are different, inasmuch as the slits of the thermopile and colli- 

 mator were narrower in the case of the sun than in the case of the 

 electric light. To make an absolute comparison, the ordinates of 

 the solar curve must be multiplied by 4 - 5. It will be seen that, 

 generally, our curve corresponds with Langley's, the difference 

 in position of the maxima and minima being in no case greater 

 than one-sixth turn of the screw of the thermopile. His radiation 

 receiver was so much narrower than ours, that his readings would 

 show the rises and depressions more exactly. A comparison of the 

 curves will show that almost all the atmospheric absorption is 

 due to watery stuff. The depression shown in Langley's and our 

 own solar curves at 8660 is not coincident with water. His 

 shows the water-band at 8240 (which is that shown in the photo- 

 graphs), and, no doubt, we missed it owing to the steepness of the 

 descent from X7900. The dip at 8660 is probably due to a hydro- 

 carbon, to which we will not here refer further in the present com- 

 munication. We wish to emphasise the fact that the "a" group, 

 corresponding to X7200, is not due to water as a liquid ; nor when we 

 look at Diagram I and see the absorption of 2 feet of water, can we 

 suppose that in the dip from 7300 to 7600 the A line is included. 



Professor Langley in his paper states that the atmospheric absorp- 

 tion in the infra-red of the spectrum is comparatively small, and he 

 gives a hypothetical thermogram of the extra-atmospheric solar 

 spectrum in which the maximum energy is in the yellow, and the 

 energy curve descends steadily on both sides of this maximum. We 

 do not think our experiments quite confirm his views in this respect. 

 vol. xxxv. z 



