288 



Lord Rayleigh. On the Intensity of 



[Nov. 18, 



Prism of Crown Glass (I), Remounted; 





Lord Rayleigh. 



Mr. Gordon. 



Mean. 



Aug. 24 



0-04085 . . 



0-04100 . . . 



. 0-0409 



„ 25, morn.. . 



0-04183 . . 



0-04199 . . 



0-0419 



j, 25, even.*. 



0-03950 . . 



0-04030 . . . 



0-0399 



,, 26 



0-04190 . . 



. . 0-04170 . . . 



0-0418 



Mean. . . . 



0-04102 . . 



0-04125 . . . 



0-04113 



The difference between 0-04113 and the mean previously found, viz., 

 0*04095, has no significance. 



In consequence of the detection of a greatly augmented reflection 

 from another glass surface (II, below), as the result of a repolish 

 with putty powder, this surface also was submitted to similar treat- 

 ment. Immediately afterwards, on August 30th, a much increased 

 reflection was observed, the numbers by two observers being — 



0-0481, 0-0472 ; mean, 0*0476. 



The disk gave, as before, a transmission 0*04119 ; so that the num- 

 bers for the repolished face depend too much upon the assumed effect 

 of various obliquities of the inclined plate to be fully trustworthy, 

 even were they sufficiently numerous to guard against accidental 

 errors. But they proved, unequivocally, a considerable increase in 

 reflecting power as the result of the repolish. 



In view of these results, a new disk was prepared of angular aper- 

 ture about 17^°, and, consequently, with a transmission equal to 

 0*04763. The numbers obtained with this are shown in the following 

 table : — 



Prisms of Ground Glass (I) Repolished. 



Lord Rayleigh. Mr. Gordon. 



Aug. 30, aft. ? 0-0461 .... 0-0451 



„ 31, morn 0'0451 .... 0'0454 



„ 31, aft 0*0448 .... 0*0447 



Mean 0*0453 .... 0*0451 



Final mean= 0*0452. 



The observed result now agrees remarkably well with that calculated 

 from Fresnel's formula ; but unfortunately it depends more (for about 

 5 per cent, of its value) than could be wished upon the use of the 

 oblique plate. 



* The considerable discrepancy shown in this set of readings was probably 

 caused by insufficiency of light. 



