[ 361 ] 
IX. Expey'iinental Investigations on the Effective Temperature of the Sun, made at 
Daramona, Streete, Co. Westmeath. 
By William E. Wilson, M.R.I.A., and P. L. Gray, B.Sc., A.R.C.S., Lecturer in 
Physics, Mason College, Birmingham. 
Communicated by G. Johnstone Stoney, E.R.S. 
Received January 4,—Read March 15, 1894. 
The expression “ effective temperature of the sun ” has by this time obtained a well- 
defined meaning, and may be taken (as stated by Violle and other physicists) to be 
that uniform temjierature which the sun would have to possess if it had an emissive 
power equal to unity, at the same time giving out the same amount of radiant energy 
as at present. 
The older estimates of this quantity were little more than guesses, and varied 
between 1500° C. and 3 to 5,000,000° C., or more. 
The former of these values was given by assuming that Dulong and Petit’s 
formula 
R = mol, 
where R = intensity of radiation, t = the temperature of the radiating surface, and 
m and a are constants for any one substance, held up to any limit. 
The result given by it is obviously too low, as it is less than even the melting-point 
of platinum, the vapour of which probably exists in the solar atmosphere, and consider¬ 
ably lower than the temperature which may be obtained in the focus of a large lens. 
The higher values were found by using Newton’s law, in which radiation is taken as 
simply projiortional to difference of temperature between the radiating body and its 
surroandings, a law which is proved to hold good only for very small differences. 
It would appear, then, that by far the greatest difficulty in estimating the value of 
the solar temperature arose from ignorance of the law which connects the radiation 
from a hot body with its temperature, although there are minor difficulties wdiich may 
still produce uncertainties in the final result. 
One thing seems certain, that the temperature of the sun is far higher than any 
we can produce in our laboratories. This being so, the best that can be done is to 
make direct determinations of the connection between radiation and temperature 
within the widest possible limits, find an empirical law to which the observations 
MDCCCXCIV.—A. 3 a 14.9.94 
