Undiffracted Long-waved Heat Rays by Wire Gratings, 32 7 



17 to 13 mm. in order to increase the deflexions. Two series 

 of experiments showed good agreement, and gave 100 /x as the 

 mean wave-length of the radiation. The non-homogeneity of 

 the radiation was not much greater than that produced by 

 introducing the thicker quartz plate. 



§ 6. The experimental arrangement for Reststrahlen is 

 shown in fig. 1. A denotes the Welsbach mantle, which 

 serves as the radiating source, placed in a case closed on 

 three sides ; opposite the fourth side is arranged a movable 

 rocksalt plate D, which serves as a trap- door. The rays fall 



Fi*. 1. 



directly on the silver concave mirror B, then at the polarizing 

 angle on the plane mirror E, consisting of selenium in the 

 experiments on fluorite Reststrahlen, and quartz in those of 

 rocksalt. Both substances are sufficiently transparent for 

 the rays in question, and possess a very high index of re- 

 fraction*. They are particularly suitable, therefore, as 

 reflecting polarizers. 



After the rays have been linearly polarized in the horizontal 

 plane by reflexion at the mirror E, they pass through the 

 wire grating P, which is mounted on a revolving support C. 

 This support has already been completely described in our 

 previous contribution (I. c. § 4). It consists of a hori- 

 zontal brass tube, capable of orientation about its axis of 

 revolution, cut at right angles at one end, and at an angle 

 of 44°*4 at the other. To this latter end a brass plate Q 

 with a square opening is fixed, which can be orientated about 



* Selenium mirrors were first used as polarizers by A. Pfund, Johns 

 Hopkins Univ. Circular, iv. p. 13 (1S06). This element shows only a 

 slight dispersion in the infra-red, for there its optical refractive index is 

 equal to the square root of its dielectric constant. A selenium plate of 

 0*5 mm. thickness transmits 35 per cent, fluorite Reststrahlen. Quartz is 

 still considerably more transparent for rocksalt Reststrahlen. 



