OF RADIANT HEAT THROUGH DIFFERENT BODIES. 19 



posed to a radiation of 30° of tlie thermomultiplier, liave furnished the 

 following results : 



Order of the screens. 



Deviations of the 

 galvanometer. 



1. Translucid .';-38 



2. 6-50 



3. 8-66 



4. Dull 12-58 



5. 14-79 



6. Slightly dull 17*42 



7. Transparent 18-79 



8. 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. galvanometer. forces. 



2-068 21-625 21-850 



4-136 20-312 20-343 



6-202 19-687 19-687 



8-272 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 M'ith 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 ra.ji 

 to the incident rays. The difference between each of these quotients and 

 unity will give the corresponding loss ; that is, the proportional part of 



c 2 



