Undijfracted Long-waved Heat Rays by Wire Gratings. 329 



that used by Prof. Wood and one of us. The Invert- mantle 

 A, the screen C with diaphragm B, the quartz lenses L 1; L 2 

 with mounted central screens cc x and a 2 , the radiomicrometer 

 M and the thin glass disk trap-door D are identical with 

 those of the former arrangement. The screen with diaphragm 

 F situated between the lenses Lj and L 2 bas, how r ever, been 

 altered as follows : a brass disk H with a circular diaphragm 

 F * is capable of motion in a circular groove G. The wire 

 grating P can be attached securely to the plate H by means 

 of clampscrews. Finally, the rotating support, denoted in 

 fig. 1 by 0, is placed between the lens Li and the diaphragm 

 F ; to avoid complications this is omitted in fig. 2. 



With regard to the details concerning the arrangement of 

 the quartz lenses reference should be made to the papers 

 already cited. It is only necessary to mention here that in 

 taking advantage of the extraordinary difference in the 

 indices of refraction of quartz for heat rays on both sides of 

 the region of absorption (1*50 to 2*14), the diaphragms and 

 lenses were so arranged that the rays of large w T ave-length 

 emitted through the diaphragm B were united by means of 

 the lens L : at the diaphragm F, and later on concentrated 

 on the microradiometer by means of the lens L 2 (dashed 

 lines). The ordinary short-waved heat rays, which form a 

 divergent pencil behind the lens L t , were held back partly 

 by the plate H and partly by the central screen u x (dotted 

 lines). By introducing a rocksalt plate 10 mm. thick in 

 their path, it was proved that long-waved rays of approx. 

 100 fi were free of short-waved impurities. 



§ 8. As polarizer we applied first of all quartz plates. We 

 w T ere soon convinced, however, that one of our finest metal 

 wire gratings was a much more complete and convenient 

 polarizer, the intensity being also stronger. For this reason 

 the platinum grating Pt la was always kept fixed to the 

 plate H and used as a polarizer. This convenient rotating 

 polarizer was of very great advantage. 



The observations were then carried out in such a way that 

 by means of the rotating support C the investigated grating 

 was inclined to the path of the rays, and a number of de- 

 flexions measured for two positions of the polarizer : 1, when 

 the wires of the gratings were parallel, and 2, when they 

 were at right angles to one another. The investigated grating 

 was then removed and the deflexions again measured in both 

 positions of the polarizer. These, on account of circum- 

 stances which are treated later on due to the fact that the 



* This should in fig. 2 be more to the right, at the point of section of 

 the rays. 



