374 
MESSES. W. E. WILSON AND P. L. GRAY 
burner, or by heating oxychloride of magnesium in the oxyhydrogen flame, pre¬ 
liminary experiments having been made on the emissive power of the various 
substances at these high temperatures. 
Some little doubt must necessarily exist as to the power of knowing exactly what 
these temperatures actually were; nevertheless, the results obtained appear con¬ 
sistent and trustworthy, and the accuracy of the parabolic formula was tested 
satisfactorily up to a temperature of something like 2,000° C. 
In our experiments, the heat from the platinum strip was, with our first meldo- 
meter, allowed to fall on a mirror of sjDeculum metal at 45°, and thence into the 
radio-micrometer. The temperature of the platinum was raised step by step, and, at 
each step, the deflections, both of the temperature scale and of the radio-micrometer, 
were noted. 
Numerous sets of experiments were made, but with some want of uniformity in the 
results. At first it appeared that Stefan’s"^ law of the fourth power'expressed the 
results; then, with additional precautions, E^osetti’s law appeared to be confirmed. 
But the want of knowledge as to the reflective power of the speculum metal, with the 
alterations in the state of its surface, as well as difficulties in throwing the reflection 
of the glowing platinum fairly into the radio-micrometer, prevented our acceptance of 
any of these results as beyond suspicion. 
With the second meldometer, the need of a mirror was obviated; the differential 
radio-micrometer was replaced by one of the ordinary single form, perfectly protected 
against accidental radiations, and, finally, three independent series of experiments 
gave concordant results which may be very closely expressed by o, fourth 2 ^oiver law. 
The radiation is taken as proportional to the deflections on the scale of the radio¬ 
micrometer, which was at a distance of about 123 centims. ; the extreme angular 
deflection was about 20°, and up to these limits the proportionality is proved to hold 
accurately.! 
The curve (fig. 6) is calculated from the formula 
II = « (in _ i^r)^ 
where 
R the radiation expressed in scale-readings, 
T = the absolute temperature of the incandescent platinum, 
Tq — the absolute temperature of the n:iedium surrounding the radio-micrometer 
{i.e., temperature of the room), 
and 
a is a constant which was calculated from four points on the experimental curve. 
In this case, log a = 11'67868. 
The temperature of the room being about 15° C. = 288° absolute, then R = 0, 
T = Tq = 288°, will give a point both on the experimental and the calculated curves. 
* Stefan, ‘Wien. Ber.,’ vol. 79, (1), 1879, p. 391. 
t See p. 378. 
