ox THE EFFECTIVE TEMPER.4TURE OF THE SUN. 
377 
and the determination of his high temperature appears to be wanting in certainty. 
He finally obtains results which do not agree with any formula hitherto given. 
The least disagreement is found with an empirical expression given by Weber, 
but Paschen’s curve (in which, as in our own, the abscissae ar-e temperatures, and the 
ordinates radiation) falls nearly as much below AVeber’s as it rises above Stefan’s. 
Taking, as a particular instance, Paschex’s observed radiation at 1273° and 1673° 
(absolute) = 69 and 295 approximately, the fourth power law gives 50 and 148, while 
Weber’s gives 76 and 570. 
Paschex’s results would therefore indicate a much more rapid rise in radiation 
than that indicated by our fourth power law; in the case just quoted the exponent 
would be about 5’3. 
We are supported, however, in our adoption of the fourth power law, not only by 
our own and Stefax’s results, and Lecoxte Steyexs’ conclusions, but also by some 
work of ScHXEEBELijt and in a very interesting way by an investigation of 
MAXx’s,| who deduces the law from the electro-magnetic theory of light.§ 
On the whole, therefore, we think there can be little doubt that, at least in the 
case of incandescent platinum, the increase of radiation with temperature may be 
most accurately expressed by the fourth power law, and that the divergent results 
obtained by different investigators are chiefiy due to want of certainty in the deter¬ 
mination of high temperatures, and in a less degree to complication of apparatus, with 
its accompanying accumulation of small errors. In the case of our own experiments, 
the temperature of the platinum strip is known with a doubt of only some 6° C. at a 
temperature of 1500° C. ; the radiation falls directly on the radio-micrometer, and the 
proportionality of the deflections of the latter to the radiation falling upon it is 
strictly demonstrated by experiment. It would seem, therefore, that the results 
cannot be far from the truth, which conclusion is largely strengthened by the 
confirmations already mentioned. 
It has been generally assumed that the deflections of the spot of light on the scale 
of the radio-micrometer are proportional to the amounts of radiation falling on the 
receiving surface of the instrument. In the above experiments the extreme deflec¬ 
tion was about 20°, and it therefore seemed necessary to determine by direct 
experiment whether this proportionality held up to this high limit or not. This was 
done in the following manner :— 
A cube of boiling water was supported at a distance of about 80 centims. from the 
* H. F. Weber, ‘ Berlin Akad. Ber.,’ 1888, 2, p. 933. 
t ScHXEEBELi, ‘Wiedemann’s Annalen,’ 1884, vol. 32, p. 403. 
+ Boltzmann, ‘Wiedemann’s Aoinalen,’ 1884, vol. 32, pp. 31 and 291. 
§ [It must be noticed, bowever, that both Stefan’s and Boltzmann’s results -were supposed to apply, 
strictly speaking, to “pure” radiation from a surface of unit-emissive power, so that the agreement 
must not be insisted on too strongly. All we can say certainly is that, for the particular results of 
particular experiments, the fourth power law is found to hold very accurately, and has therefore been 
adopted.] 
MDCCCXCTV. — A. 3 C 
