Intelligence and Miscellaneous Articles. . 399 



These numbers, which also require a special revision, prove that 

 the passage of radiant heat under the present conditions is just the 

 opposite of the foregoing : — 



I. Decreases the more the angle of inclination increases. 



II. That this decrease is greater the greater the number of plates. 

 In the year 1854 Professor Schweigger* showed that an object 



seen through several plates appears brighter if an acute position is 

 given to the plates, than if it is looked at through the smallest diameter. 

 In this manner the fact found for heat was shown for light. 



In making his experiment, Schweigger had been led by an obser- 

 vation mentioned by Schiilenf, according to which the image of a 

 telescope is made more distinct by placing the object-glass at an 

 acute angle ; this he proposed to strengthen by several glasses ac- 

 cording to the principle of the electromagnetic multiplier, " to form 

 from it a lecture experiment." Any further explanation or proof of 

 a connexion with other optical processes is not met with J. 



To ascertain, if possible, such a connexion, and also to establish 

 the limits of the phenomenon as far as it overcomes the counteract- 

 ing absorption of the irradiated substance, the author has since then 

 again taken up the subject. 



It has been established without any doubt that the maximum in- 

 tensity of thermal as well as luminous rays traversing colourless glass 

 pieces is attained at the polarizing angle — that is, at an inclination of 

 about 55° to the normal ; and from that point a rapid and increasing 

 diminution sets in. 



Thus with, twelve glasses, in the passage from 50° to 55° there 

 was an increase of the thermal action (which for measurements is 

 far more certain than the optical estimation) from 10*32 to 10*60 — 

 that is, 3 per cent. ; from 55° to 60°, a diminution of 10*60 to 9*25— 

 that is, 1*3 per cent. 



With rays whose plane of polarization is at right angles to the 

 plane of refraction of the glass piece, under the same angle of incli- 

 nation, the deflection of the galvanometer-needle increased from 9° 

 to 9°*5 — that is, 6 per cent. ; with a continued inclination it decreased 

 from 9°*5 to 8°*75, — that is, 8 per cent. Hence the increase in this 

 case is greater, the decrease smaller than in the previous case. 



When the original plane of polarization and plane of refraction of 

 the plates traversed coincided, the diminution of intensity continued 

 during the further inclination, although to a diminished extent. 

 With the same twelve glasses it amounted, for the change of angle 

 from 50° to 55° to 2*19 to 1*19 — that is, 46 per cent. ; for the^change 

 from 55° to 60°, from 1*19 to 0*76 — that is, a change of 34 per cent. 



On coloured glasses or other substances, such as rrjica, isinglass of 

 various colours, bluish-green transparent gold lamellae, along with the 

 phenomenon mentioned, absorption was observed in a decided manner. 



With plates of yellow glass or of mica it was manifest how the 

 increase of intensity with increasing inclination, and the simulta- 



* (i On the Optical Significance of the principle exhibited on the Elec- 

 tromagnetic Multiplier for strengthening the Magnetic Swing," Abhand- 

 lungen der naturforschenden Gesellschaft zu Halle, vol. xi. p. 201. 



t Contributions to the Dioptrics and History of Glass. By Max. Lud- 

 wig ChristofF Schiilen, Pastor of Esslingen in Swabia. Nordlingen, 1782. 



X A communication recently published by Prof. Magnus, which, according 

 to a private communication, treats the same subject, has not reached the author. 



