228 



KNOBLAUCH ON RADIANT HEAT. 



was placed, and approximating the latter to it until the deflection 

 of 35° was produced by the combined action of the rays emitted 

 from and passing through the ivory plate. As soon as the needle 

 had rested at this point, the diathermanous substance to be 

 tested, e.g. the red glass, was introduced on the opposite side of 

 the diaphragm before the thermal pile. We were already aware 

 that the rays from a body below 212° F., which had directly de- 

 flected the needle to 35°, after their transmission through red 

 glass produced a deflection of 10°-10°-25, and that of the Ar- 

 o-and lamp, with the same direct action, after their transmission 

 through the glass, a deflection of 21°-75 (see Table XVII. and 

 p. 229). Thus when a portion of these rays joined the former 

 so as conjointly to produce a direct deflection of 'db°, if the me- 

 thod was sufficiently delicate, the instrument under these cir- 

 cumstances should indicate a different deviation from 10°'25 

 when the red glass was inserted in the same spot. This was 

 really the case. A deflection of 13°-G2 was produced. 



The same result was obtained in every other instance. Thus 

 it amounted to 16°*75 when ivory was replaced by black opake 

 lac, and even ll°-62 when a metallic plate perforated with two 

 needle-holes was used instead of this. 



The less the first diathermanous screen interrupted the heat 

 which the second transmitted, the higher the deflection ought to 

 be after the insertion. Thus with the same red glass the needle 

 only receded to 27°'5 when the first screen consisted of colour- 

 less glass 1*9 millim. in thickness. 



The following table represents the great differences which also 

 occurred in the case of the other diathermanous media. (It con- 

 tains the arithmetic means of every two observations.) 

 Table XVII. 



