TJndiffracied Long-waved Heat Hays by Wire Gratings. 331 



employed, ascertained by means of a dividing machine ; 

 their surfaces were rectangular in shape, of sides 20-30 mm. 

 They were all previouslv optically investigated by means, 

 of a spectrometer and sodium light. The rela ive quality of 

 the refracted image, which characterizes the uniformity of 

 the grating and hence its structure, is further roughly 

 indicated by the interposition of one or more exclamation 

 marks (!) in Table I- In addition to the values of a and b, 

 corresponding to /3 = 0, we have calculated those of the 

 apparent grating constants and slit-widths a' ', b' respectively 

 for angles of inclination increasing by 5° from 5° to 60°, 

 where b' finally disappears. 



III. Transmission through Wire Gratings. 



§ 10. We will describe the properties of our gratings in 

 the same order as in the previous paper, from which Table J I. 

 is extracted besides forming a continuation of that table 

 (I. c. § 12). At that time the wave-lengths were determined 

 by measurements with a fluorite prism*; later observations! 

 proved them to be a little too large ; the error is not con- 

 siderable up to 3 fi, scarcely more than 2 per cent. ; for 5 /x, 

 however, it is about 10 per cent., so that we have now 

 ascribed the proper value to the wave-length 4'5/x. 



Table II. contains for each of the five gratings the ratio n 2 

 of the transmissibility of polarized rays for /3 = 0° and 45°'6, 

 beginning with a number of the previously investigated 

 wave-lengths from 0'49/a (F line) to 4'5 fi. Then follow 

 those for fluorite Reststrahlen (24 p), rocksalt Reststrahlen 

 (52 //,), isolated rays through quartz lenses (100 /a), the same 

 after further filtration through a quartz plate 4 mm. thick 

 (108 fi), and, finally, the long waves of the quartz-mercury 

 lamp (about 314 /z). 



By referring to the table we see that the gratings — with 

 one exception — polarize in the Hertzian sense after passing 

 the neutral inversion point where Q s — Q p (n 2 — 1); n 2 de- 

 creases more and more with increasing wave-length, in such 

 a manner that it practically vanishes at 100 /x when the 

 grating is inclined at 45°" 6, while with Pt 1 and Cu 1 the 

 JL transmission only amounts to 3 and 5 per cent, respectively. 

 The purer and somewhat longer waves (108 /x) become with 

 these gratings up to 1 and 3 per cent, completely polarized. 

 For the longest waves (about 314 /z), the polarization is in 

 every case complete. 



* H. Rubens and W. Snow, Wied. Ann. xlvi. p. 530 (1892); see I. e. 

 Table I. 



t H. Rubens, Wied. Ann. li. p. 381 (1894) ; F. Paschen, Wied. Ann. 

 liii. p. 301 (1894). 



