Focal Isolation of Long Heat- Waves, 

 Table II. Quartz perpendicular to axis. 



257 



d. 



D'. 



* 



7i- 



mm. 



2-00 



4 03 



728 



11-74 



14-66 



18-69 



per cent. 

 81-3 

 66-4 

 49-8 



3f>-:> 



29-0 

 22-6 



0-103 

 0-102 

 0-096 

 0-088 

 0-084 

 0-079 



o-ioo 



0-089 

 0-075 

 0-069 

 0062 



The mean wave-length of the radiation under the con- 

 ditions which obtained in the experiments on reflexion and 

 transmission was 108 /a. These were made with a 1 mm. 

 quartz plate always in the path o£ the rays. From Table II. 

 we find that, for such a plate q l = m 089 for our 108 jn rays, 

 while for residual-rays of various wave-lengths previous 

 observations have given the following values : — Sylvin (6 i £t) 

 g 1 = '2Sl, potassium bromide (82 ft) q ± = '216, potassium 

 iodide (97 /x) ^ = -104 — . 



We find also that black paper, and even black cardboard 

 are partially transparent for the long waves, while films of 

 smoke, so thick that the sun cannot be seen through them, 

 are almost perfectly transparent. The smoke was deposited 

 on a thin film of mica, the absorption and reflexion of the 

 mica being eliminated by making observations with the 

 covered and uncovered portions. The results are given 

 in Table III. for : 1, thin black tissue-paper ('025 mm.) 

 nearly opaque to light ; 2, thick black paper such as is used 

 for packing photographic plates ('11 mm.) ; 3, black card- 



Table III. 



Radiation. 



Wave- 

 length A. 



Transmission D in per cents. 



Paper I. 



Paper II. 



Cardboard. 



Smoke. 



Isolated by spectrometer... < 



Residual rajs from fluorite ... 

 ,, „ rock-salt. 



(i. 



2 



4 



6 



12 



26 



52 



108 



per cent. 







0-9 



1-7 



8-2 

 24-2 

 460 

 61-5 



per cent. 















1-4 



3-2 

 15:1 

 335 



per cent. 

 

 

 

 

 

 

 16 



per cent. 



S-(> 

 160 

 37 -0 

 76-7 

 91-3 

 91-5 





Phil. Mag. S. 6. Vol. 21. No. 122. Feb. 1911. 



