ox THE EFFECTIVE TEMPERATURE OF THE SUN. 
373 
in the visible spectrum, there was no regular reflection from the back surface. Even 
if the black varnish happened to possess a refractive index equal to that of the glass, 
the virtual effect would merely be a slight thickening of the plate, and it would 
still hold that all the energy due to what we may call for brevity, the “ visible wave¬ 
lengths,” reaching the back surface, was there absorbed and then diffused in every 
direction, the amount reaching the radio-micrometer on this account being absolutely 
negligible. 
As for the ultra-red vibrations, it woidd be unreasonable to suppose that when all 
the “ visible wave-lengths ” were absorbed, there should be a rapid change in the 
nature of the back-reflections, so that a “dark image” might be reflected when no 
sign of a “ light image ” was to be found. Moreover, if such a condition could be 
considered likely, the additional radiation must be extremely small, as we know that 
by far the greater portion of the heat-energy of the solar radiation is contained 
within the limits of the visible spectrum. 
The point hardly needed further confirmation, but as a check on the curve (fig. 9), 
obtained from Fresnel’s formula, we made three photometric obseiwations, as 
mentioned elsewhere (p. 386), which gave points very nearly on the theoretical curve. 
On the Law Connecting Hadiation and Temperature. 
We have already mentioned some experiments which have been made in this part 
of the subject, and seen that it is ignorance of the law which has been the main 
cause of disagreement in the final estimation of the solar temperature. 
Rosetti’s experiments on this point were divided into two parts. He first found 
the effect on his thermopile of the radiation from a cube filled with water, and 
afterwards with mercury, at temperatures from about 60° to 300° C. He then found 
an empirical formula which closely expressed the observed results. The law is 
expressed thus— 
y ^ ccT- (T - 9)-h (T - 9), 
where 
y = the thermal effect of the radiation as given by the deflections on the scale 
of the thermopile, 
T = the absolute temperature of the radiating body, 
9 = the absolute temperature of the medium surrounding the body on which 
the radiation falls; 
while 
a and 6 are constants which must be determined from two corresponding values 
of y and T. 
Experiments were then made with the radiating body at higher temperatures, 
which were obtained either by holding a disc of metal in the flame of a Bunsen 
