1886.] Light reflected at nearly Perpendicular Incidence. 



289 



Prism of Grown Glass (II). 



So soon as it appeared that the reflection from the face of prism (I) 

 fell so much short of what was to be expected in accordance with 

 Fresnel's theory, I tried another prism whose surface was still older 

 than that of (I). The event proved a still more marked deficiency. 

 With the aid of the disk giving transmission 0*04119, the following 

 numbers were obtained : — 



Prism (II), before Repolishing. 

 Lord Rayleigh. Mrs. Sidgwick. 



Aug. 26. . 0-0349 — 



„ 27.. 0-0342 .... 0-0350 



Mean. . 0*0346. 



Although somewhat dependent upon the assumed effect of the oblique 

 plate, this number is far too low to be consistent with anything 

 obtainable from Eresnel's formula with an admissible index.* This 

 circumstance suggested a repolishing of the surface, which, however, 

 was superior to that of (I), so far as could be judged from close 

 inspection in a favourable light. The repolishing was executed by 

 Mr. Gordon by means of a disk of wood charged with putty powder 

 and mounted in the lathe. Observation now demonstrated a remark- 

 able improvement in the reflecting power, as the following numbers 

 will show : — 



Prism (II), after Repolishing. 

 Mrs. Sidgwick. Lord Rayleigh. 



Aug. 28, morn.. . 0-0491 0'0488 



„ 28, aft — .... 0-0479 



„ 30, morn.. . — .... 0*0484 



Mean. . 0*0483. 



Here again too much depends upon the oblique plate, the transmis- 

 sion of the disk being only 0*04119 ; but there can be no doubt of an 

 increase in the reflection of something like 30 per cent. If we may 

 argue from the number obtained from prism (I) after repolishing, 



* If e 2 be the proportions of light reflected at incidence 0, then Fresnel's formula 

 is equivalent to — 



1— e 



tan 9 1 = tan 0, 



1 + e 



* 



by which 2 is found. The index is then given by sin 0/ sin 0j. In the present case 



e 2 = 0-0346, e = 0*1860, (1 - e)j (1 + e) = 0-08140/1*1860, 

 so that, since = 13° 52', 6\ = 9° 37', fi = 1*434. 



Mr. G-ordon. 

 0*0344 



Mr. U-ordon. 



0*0473 

 0*0481 



