380 
^lESSRS. W. E. WILSOX AXD P. L. GRAY 
The simple form gives a great saving of time in calculating out the results of the 
observations, and we generally adopted it in the course of our work. The only 
direction in which wm can look for an exjdanation of the great difference between 
JiOSETTi’s law and our own, is in that of his method of estimatino- his hio^h 
temperatures, which appear to be somewhat uncertain, whereas v>'e can feel confident 
in the accuracy of our own method to witlhn ff: 6° at 1500° C. The chances are 
that his discs of metal were at a lower temperature than that assumed (but not 
measured) by him; and if that were so, the differences between his results and ours 
would be in the direction in which we find it. 
The Emissive Power of Platinum at High Temperatures. 
ScHLEiERMACHER* and PtOSETTit have made experiments on this subject which 
at first sight appear to disagree, but on examination confirm one another in an 
interesting manner. From the curves which SchleiermaCher’s results give, we 
obtain the emissions at certain temperatures (l) from polished platinum, (2) froui 
platinum covered with black oxide of copper, which may be assumed as approximately 
the same as that from a lamp-black surface. The fourth column in the following 
table gives the ratio of the two emissions :— 
Absolute 
temperature. 
Emission. 
Ratio 
Plat, (black). 
Plat, (bright). 
bright 
O 
300 
65 
12 
5-42 
400 
96 
20 
4-80 
500 
147 
34 
4-32 
600 
220 
52 
4-23 
700 
317 
77 
4-12 
800 
445 
112 
3-97 
The figures in the fourth column show a gradual fall in the ratio as the temperature 
rises. PtOSETTi, at an absolute temperature of about 1500°, found for tlie ratio 
100/35 = 2‘9, which falls in fairly satisfactorily with a theoretical continuation of 
Schleiermachee’s results. As it is impossible, with our present arrangement of 
apparatus, to keep the platinum lamp blacked at a high temperature, and as the ratio 
is evidently altering very slowly near the point at which Rosetti made his deter¬ 
minations, we shall use his ratio in calculating our results, be., we shall take 
Emission from lamp black 100 
Emission from bright platinum 35 
* ‘WiED. Ami.,’ 1885, vol. 26, p. 287. 
t ‘ Pliil. Mag.,’ vol. 8, 1879, p. 445. 
