ON RADIANT IIEAT. 
349 
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 unpolarized rays, were in¬ 
creased. 
No difference could be detected between the cases when the rays passed 
along the axis of the crystals, and the plane of polarization wus respectively 
horizontal or vertical. 
M. Knoblauch now proceeded to try whether the rays which exhibit 
quantitative differences as above, would show qualitative differences in the 
same cases; that is, differences in the power of transmission through diather- 
manous bodies. 
The diathermanous bodies employed for screens were blue, yellow, red 
and green glasses. 
After transmission through a brown vock crystal, the proportion of rays 
penetrating the different screens differed in the two positions, parallel and 
perpendicular to the axis, only within errors of observation. 
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 ami red glasses, 
rather less through the blue, and least through the green. 
Another set of experiments, in which the plane of polarization was hori¬ 
zontal, 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 differ¬ 
ence is very small, for yellow rather greater: the author infers a real dif¬ 
ference. 
With polarized light and with the plane of polarization vertical, the differ¬ 
ences 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 differences with the 
yellow and blue glasses, and in an opposite ratio in the respective cases. 
W ith tourmaline exactly similar results were obtained in the corresponding 
With dichroite also the author says, “so far as the examination extended, 
qualitative differences dependent on the direction of transmission have also 
boen observed.” 
At the conclusion the author makes some observations in explanation of 
‘he phaanomena: these may be more briefly and clearly expressed thus:— 
file unpolarized rays incident along the axis of the crystals examined or 
parallel to it, undergo a certain absorption dependent on the nature of the 
crystal; and this is no further modified, since in this direction there is no 
double refraction. 
W if the ray be incident perpendicular to the axis, it is divided into two, 
oppositely polarized; and these are differently absorbed. 
I hut which has its plane of polarization parallel to the axis, has the same 
^sorption as along the axis. That which has its plane of polarization per¬ 
il ‘nilicular to the axis is absorbed more or less than the former, in different 
' n different crystals, and for the different component rays, 
hen the unpolarized rays are incident, then the result is compounded 
the se separate effects. 
sin gly 1 ra ’ VS prcviously P olarizc(l ar e employed, the effects are displayed 
