386 
MESSRS. W. E. WILSON AND P. L. GRAY 
Sir J. Conroy'* has shown that the curve drawn from Fresnel’s formula is verified 
by experiment to within j per cent, at the angles of incidence generally used in our 
observations. 
The angle of incidence is obtained at each experiment by finding the distance 
between the end of a certain steel rod in the heliostat and a collar which slides alono- 
it; the angle corresponding to any distance could be found by means of a curve, 
which it is unnecessary to give here. 
(3) The ratio of the radiation outside our atmosphere to the amount which reaches 
the earth. This is obtained by calculating the altitude from the known declination, 
hour angle, and latitude, and taking the percentage of absorption from the curve 
(fig. 8) which we have already discussed. 
(4) The ratio of the emissivity of bright platinum to that of a lamp-blacked 
surface, which, as already mentioned, we take as 35 :100. 
To take a typical case :— 
Date, Sept. 4th, 1893. O Declination = 7°T N. 0-1 diameter = 15''9. 
Time, 10*^ 54“, local. Therefore O altitude = 41°'8. 
Balancing temperature = 1514° absolute. 
By curve (fig. 8) absorption = 36 per cent. 
Therefore transmission = 64 per cent. 
Diameter of O = 31'‘8 = 0°'53. 
Therefore 
Area of platinum_ /4-702Y _ 
Area of srui \ ’53 / ^ 
Angle of incidence on glass = 61°. 
Therefore amount of heat reflected = 9‘5 per cent. 
Ratio of emissivity of platinum and lamp black = 
Therefore the radiation from the sun is 
100 100 
X = 453'1 
that of the platinum at a temperature of 1514° absolute. 
The temperature of the sun is therefore 
1514 X ^453T = 1514 X 4-614 = 6985° absolute, 
according to this single observation. 
It was not only necessary to take observations with the sun shining (A) into the 
lower circuit and (B) into the upper circuit, but, on account of possible differences in 
the state of the surfaces, back and front, of the copper foil receivers, it was esseiitial 
* ‘ Phil. Trans.,’ 1889, (A), vol. 180, p. 245. 
