Intelligence and Miscellaneous Articles. 477 



the other filled with well-dried coal-gas, the lamp lit, and by means 

 of the rheostat the needle reduced to zero. Equilibrium being set 

 up, the lamp is masked by means of a semicylindrical screen. The 

 tube originally full is exhausted, the other filled with gas, and the 

 screen removed, on which the needle begins to move. The deflec- 

 tion may easily amount to several degrees, and can be readily seen 

 in projection. 



The differential method is readily[applied to the investigation of the 

 rotations which the plane of polarization of a ray of heat undergoes 

 under the action of a substance which has but little activity. The 

 polarized ray is divided by a plate of calc-spar whose principal section 

 makes an angle of 45° with the original plane of polarization. The 

 two images have in that case the same intensity ; one falls on the first 

 pile, and the other on the other ; and the needle is brought to zero. 

 If, then, the original plane of polarization is deflected by a very small 

 number of degrees (a, for example), a difference is established be- 

 tween two images that were originallyequal to one another and to I, 

 the measure of which is 



I [sin 2 (45 + a) — cos 2 (45 + «)] = I[4 sin 2 45 sin a cos a] = I sin 2a. 



The corresponding deviation might be still quite appreciable, while 

 even with a more sensitive galvanometer the effect produced \>-j the 

 reappearance of the extinguished image would only have led to a 

 much more feeble result. I may add that if, by means of a compen- 

 sating plate or tube, the action of which can be readily estimated, 

 the differential deflection whose relative value we have just calcu- 

 lated be made to disappear, we shall thereby obtain the value of the 

 rotation originally produced. 



I request permission from the Academy to communicate some at- 

 tempts which I have made to transport into the domain of radiant 

 heat a beautiful optical experiment devised by Biot. 



When a ray of rectilinearly polarized light, in which, for instance, 

 the vibrations are vertical, traverses at right angles to one of its faces 

 a plate of glass untempered and suitably inclined, there is no special 

 effect. If on emerging from the plate the ray passes through a plate of 

 calc-spar whose section is parallel to the plane of polarization, it is 

 totally refracted in the ordinary manner, the extraordinary image is 

 zero ; but if the plate be then made to vibrate longitudinally, and 

 if the ray traverse it in the vicinity of a nodal point, the extraordi- 

 nary image will reappear and continue as long as the vibratory 

 motion continues. I have found that under these conditions the 

 calorific image which is again formed may also be rendered visible. 



The experiment is well made in the sun. It may also be made 

 by the electrical lamp, and even with the radiation of incandescent 

 lime. But it is produced very conveniently in a manner analogous 

 to that above described — that is to say, using the vibration of a 

 plate, not to cause the reappearance of the action of a pencil origi- 

 nally extinguished, but to produce a difference of intensity between 

 two radiations whose actions on the piles of the differential appa- 

 ratus had at first been made equal. 



