RADIANT HEAT. 377 



He took in the first instance a cube of brozvn quartz, having two of 

 its sides perpendicular to the axis of the crystal. The rays of the sun 

 were reflected into a room by a heliostat, and the mirror being me- 

 tallic they remained unpolarized, and after passing through the crystal, 

 (which could be turned with its axis in different directions with respect 

 to the rays,) were received on a thermo -multiplier. The effect was 

 considerably less in the direction perpendicular to the axis. The same 

 result was found with beryl; but with tourmaline the effect was greatest 

 perpendicular to the axis. 



The rays were next polarized by a Nicol's prism, before incidence 

 on the crystal. "When the plane' of polarization coincided with the 

 axis the heat transmitted was the same in all directions. But when 

 perpendicular to the axis the differences before observed in the unpolar- 

 ized rays were increased. 



No difference could be detected between the cases when the rays 

 passed along the axis of the crystals, and the plane of polarization was 

 respectively horizontal or vertical. 



M. Knoblauch now proceeded to try whether the rays which ex- 

 hibit quantitative differences as above, would show qualitative differences 

 in the same cases, that is, differences in the power of transmission 

 through diathermanous bodies. 



The diathermanous bodies employed for screens were blue, yellow, 

 red and green glasses. 



After transmission through a brown rock crystal, the proportion of 

 rays penetrating the different screens differed in the two positions, 

 parallel and perpendicular to the axis, only within errors of observa- 

 tion. 



With the rays previously polarized (as before) sensible differences 

 were observed, the plane of polarization being vertical. 



The heat was in all cases greatest through the yellow and red 

 glasses, rather less through the blue, and least through the green. 



Another set of experiments, in which the plane of polarization was 

 horizontal, gave no sensible differences. 



Rays traversing the crystal along its axis, also exhibit no differences, 

 as was likewise the case with rays perpendicular to the axis. 



With beryl similar observations were repeated; with blue glass the 

 difference is very small, for yellow rather greater; the author infers a 

 real difference. 



With polarized light and with the plane of polarization vertical, the 

 differences are much greater with both glasses. 



With the plane horizontal no difference was found. 



Common light, passed through two cubes of beryl according as their 

 axes were parallel or perpendicular to each other, gave great differ- 

 ences with the yellow and blue glasses, and in an opposite ratio in the 

 respective cases. 



With tourmaline exactly similar results were obtained in the cor- 

 responding cases. 



With dichorite also the author says, ' ' so far as the examination 

 extended, qualitative differences dependent on the direction of trans 

 mission have also been observed." 



