OF RADIANT HEAT THROUGH DIFFERENT BODIES. 19 
posed to a radiation of 30° of the thermomultiplier, have furnished the 
following results : 
Order of the screens. Deviations of the 
galvanometer. 
CENSUE Sr) UT IR Re ol 538 
Ze ————— he ecseeveeees 6°50 
GE 8°66 
aN is PTR es 12°58 
ek dic don dudes Suteraxedlee dave 14°79 
esmeitiy Gull 2.5, .ccdecsva- sanaze 17°42 
det EAN SDATONG ns siapy nw oiiess spo shoes 18°79 
8B. = sa eersccceceeceees 19°15 
These transmissions present nothing extraordinary: the quantity of 
heat which passes through the medium is greater in proportion as the 
surface is more finely polished, as it happens in respect to light. The 
only thing to be remarked is, that in the high degrees of polish a slight 
difference produces a very slight effect. This is evident from the ob- 
servations made on Nos. 7 and 8. 
Similar processes enable us to determine the influence of thickness, 
which is one of the elements most necessary to be known in the theory 
of transmission. 
Four pieces cut out of a fine mirror were reduced with great nicety 
to different degrees of thickness in the ratio of 1, 2, 3, 4: particular 
care was taken to give to their principal surfaces a perfect parallelism, 
and the highest polish possible. The following are the deviations which 
they successively produced in the index of the galvanometer under the 
action of the same radiation, namely 30°: 
Thickness of the screens Deviations of the Corresponding 
in millimetres. galyanometer. forces. 
° 
. 2:068 21°625 21°850 
4°136 20°312 20°343 
6°202 19°687 19°687 
8272 19°375 19°375 
Each number in the second column is deduced from fifteen observa- 
tions: the quantities registered under the denomination of forces, 
representing in this particular case the respective temperatures or 
quantities of rays transmitted, have been calculated according to the 
principles with the exposition of which we concluded our general ob- 
servations. The force or temperature answering to 30°, as given by 
the table of intensities, is 35°3; now, by dividing each number of the 
third column by 35°3, we shall obtain the ratios of the transmitted rays 
to the incident rays. The difference between each of these quotients and 
unity will give the corresponding loss; that is, the proportional part of 
a eZ 
