1883.] On the Solar Spectrum and Solar Temperature. 333 



J inch of water it is distinct in the thermogram. The reason of this 

 is that when high intensities of radiation are nearly equal, an ap- 

 parent equality of intensity of image is produced in a photograph 

 when exposure is prolonged. By shortening the exposure and thus 

 sacrificing the rest of the spectrum, this band can be brought out. 

 Russell and Lapraik (see "Journal of the Chemical Society," 1881) 

 found this band visually when using a thickness of 14 feet of water, 

 apparently, therefore, the thermopile is more delicate for gentle 

 shades of radiation than is the eye. 



A 



Diagram II shows the Curves I, VI, VII, and VIII, transformed to 

 the normal scale of wave-lengths (figured I', VI', VII', and VIII' 

 respectively), from which it will be seen that the maximum energy 

 of the crater on this scale corresponds to a wave-length of about 7350, 

 or just at the limit of the red as usually visible. It will also be seen 

 that the depressions are not so marked as in the prismatic scale. 

 Diagram III gives the solar spectrum on two separate days. The 

 continuous curve was taken with the wet and dry bulb of the thermo- 

 meter standing at 68° and 78° E. respectively. According to usually 

 accepted methods of calculation, this would correspond to an eqniva- 



