362 
MESSRS. W. E. WTLSOX AND P. L. GRAY 
conform, and trust tliat no break of continuity may make an extra-polation entirely 
useless. 
So far, tlie only investigations made in this way appear to be those of Le 
Chatelier'^ and RosETTi.t Le Chatelier measured the photometric intensity of 
the red light from solid bodies heated to different known temperatures, and obtained 
an empirical law which very fairly expressed his results from 700° to 1800° C, 
He then, by passing sunlight through the same piece of red glass, measured the 
visual intensity of the “ red radiation ” coming from the sun, and, by applying the 
law just mentioned, deduced an effective solar temperature of 7600° C., which he 
admits to be an apj^roximation with a possible error either way of 1000° 
The law he found is expressed thus : 
I = 1 06'7X-3210/T^| 
where I is the photometric intensity, and T the absolute temperature of the radiating 
body. On plotting the numbers that Le Chatelier gives for corresponding values 
of I and T, it will be seen more easily than by mere inspection of the formula that I 
increases in an enormously rapid ratio as compared with T, which must evidently 
tend to vitiate the accuracy of the results obtained by extra-polation. 
Then, as Violle§ points out, it is probable that the absorption by the red glass 
decreases as the radiation increases. And in discussing a question in wdilch toted 
energy as measured by heat is concerned, it is probably better to deal by experiment 
with the total energy than with a selected wave-length, or a group of w^ave-lengths. 
Still the value thus obtained is sufficiently near those given by the utterly distinct 
methods of Kosetti and of ourselves to increase considerably the probability of the 
approximate accuracy of our results. 
PtOSETTi attacked the problem in the most direct and complete manner hitherto 
attempted. He determined a law of radiation which held well up to 2000° C., and 
found in arbitrary units the heat radiated from an incandescent bod}^ at a known 
high temperature by means of a thermopile and galvanometer. He then measured 
the heat coming from the sun in the same units, and applied Ins formula to find the 
solar temperature, which finally came out at about 10,000° C. The questions of 
atmospheric absorption and the emissive powers of his incandescent solids were 
also investigated, and his work will be referred to more than once in the following- 
pages. 
* Ee Chatelier, ‘ Compfc. Rend.,’ 1892, vol. 114, p. 737. 
t Rosetti, ‘ Phil. Mag.,’ 1879, vol. 8, 5th series, pp. 324, 438, 537. 
I The negative sign in the exponent is omitted in Le Chatelier’s paper, pvohahly by a mere slip. 
§ ViOLLE, ‘ Comjit. Rend.,’ 1892, vol. 114, p. 734. 
