138 On the Emission of Luminous Radiation at a Red Heat. 



I took a platinum plate, and having completely covered it with 

 zinc- white, I covered half the plate thus whitened with a layer 

 of oxide of iron above that of oxide of zinc ; lastly I heated it, 

 and even in this case I found the oxide of iron twenty times as 

 luminous as the other. 



Oxide of zinc is one of those bodies which at about 100° have 

 virtually the same calorific emissive power as oxide of iron or 

 lampblack. Seeing how little light, it emits at a red heat, I 

 compared at this temperature its calorific emissive power with 

 that of oxide of iron, and 1 found that it was about six-tenths 

 of that of the latter. 



Hence at a red heat the ratio of the emissive powers of these 

 two substances is far greater for heat than for light. This dif^ 

 ference, however, is bat natural, seeing that , of the total sum of 

 the rays which at redness each sends to the thermoscope, there 

 is a strong proportion of obscure rays, which have no influence 

 on the results of the photometric measurements. The thermo- 

 scopic measures refer to the total emission; the photometric 

 measure only applies to a particular class of radiations; it omits 

 all those whose wave-length is above a certain limit. 



What has been said refers merely to the effect of luminous 

 emission properly so called. The appearances change, the rela- 

 tive lustre of incandescent bodies is modified, when to the emitted 

 rays are joined the reflected or diffused rays, as is the case when 

 the surfaces observed are placed in the middle of an incandescent 

 space. These changes are easily explained, and can be readily 

 produced in the following manner. A porcelain tube is taken 

 about 0*06 metre in diameter ; it is bevelled at one of its ends ; 

 then after having closed this end by a platinum plate half covered 

 with oxide of iron, it is introduced into a furnace. The other 

 end of the tube is open, and projects from the furnace. It is 

 heated ; the plate becomes red first, and the oxide appears far 

 more brilliant than the platinum; but when the sides of the 

 tube are incandescent, the difference in lustre of the metal and 

 of the oxide diminishes or disappears ; and then in order to 

 change the function, that is, to make the platinum appear more 

 brilliant than the oxide, it is sufficient, by changing the arrange- 

 ment of the fire, to make the metallic plate distinctly less hot 

 than the side of the tube whose light can reach the eye by re- 

 flexion from the metal. 



When in making this experiment the plate which closes the 

 tube is observed with a polariscope, it is easily seen that at first, 

 when the plate alone is incandescent, the light which comes 

 from the platinum to the eye is polarized in a plane perpendi- 

 cular to that which passes through the axis of the tube and the 

 perpendicular to the plate. In the second phase of the experi- 



