1883.] On the Solar Spectrum and Solar Temperature. 339 



single temperature appears to be one having a sharply-defined maxi- 

 mum (see Diagram V). Curve A is the curve of the crater of the 

 electric light, Curve B is that of an incandescent light produced with 

 thirty-eight Grove's cells, and Curve C that produced by thirty 

 Grove's cells ; the same lamp being used in both cases. The current 

 used was 2 '25 amperes and the electromotive force 5'1 volts in the 

 last case, and in the former 2'95 amperes and 6"3 volts. (The deflec- 

 tions of the galvanometer attached to the thermopile, due to the naked 

 lights, in the two cases were as 105 to 207.) The thermogram from 

 a source of mixed temperatures would be the integration of those 

 from each temperature separately. In illustration of this we have 

 taken the three thermograms just mentioned, and transformed them 

 (see Diagram VI) to the normal scale of wave-lengths, the ordinates 

 of B being exaggerated four times, and those of C eight times. Curve 

 D is formed by making the ordinates the mean between those of A 

 and B, and the ordinates of E are the sums of those of D and C. 

 Thus E would represent the thermogram of a source composed of 

 the temperature of the positive pole of an arc lamp, an incandescent 

 lamp worked by thirty-eight cells and one worked by thirty cells, in 

 the proportion of 1, 4, and 16. In these last two curves, there is a 

 lump, so to speak, which corresponds approximately in position to that 

 shown in Langley's solar curve as corrected for J inch of water (see 

 fig. 2, Diagram IV), indicating the presence of energy due to a low 

 temperature source or sources. The mottling on the sun's disk when 

 seen in a telescope makes it evident that at its visible surface there is 

 a considerable range of temperature, and we have evidence of eclipse 

 observations that outside its ordinarily visible surface there is 

 much matter at a still lower temperature which is competent to produce 

 a continuous solar spectrum. With these amongst other facts 

 before us, we are disposed to doubt whether the extra- atmospheric 

 spectrum of the sun would give such a simple thermogram as 

 Professor Langley has suggested, and whether the maximum energy 

 would be found in the position in which he places it. We believe 

 that the curve would have a form rather more allied to our Curve E; 



At first sight it might appear hopeless to try and affix an approxi- 

 mate value to any solar temperatures, but we think that it is not 

 impossible to determine with some approach to probability the maxi- 

 mum temperature to which any compound curve is partially due, 

 more particularly when, as in the case before us, the general form of 

 the corrected curve indicates an excess in quantity of low temperature. 

 From an inspection of the curves in Diagram V it appears that for 

 any temperature higher than that of Curve A the position of the 

 maximum will be but very slightly shifted towards the more refrangi- 

 ble end of the spectrum, also that the general form of the curve must 

 be similar to that of A, and that the areas within the curves, which 



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