290 Lord Rayleigh. On the Intensity of [Nov. 18, 



under nearly similar circumstances, viz., 0*0476, we may conclude 

 that the true reflecting power of this prism is about 0*0460. 



Altogether the evidence favours the conclusion that recently 

 polished glass surfaces have a reflecting power differing not more 

 than 1 or 2 per cent, from that given by Fresnel's formula; but 

 that after some months or years the reflection may fall off from 10 to 

 80 per cent., and that without any apparent tarnish. 



The question as to the cause of the falling off, I am not in a position 

 to answer satisfactorily. Anything like a disintegration of the surface 

 might be expected to reveal itself on close inspection, but nothing of 

 this kind could be detected. A superficial layer of lower index, formed 

 under atmospheric influence, even though no thicker than toWoo inch, 

 would explain a diminished reflection. Possibly a combined exami- 

 nation of the lights reflected and transmitted by glass surfaces in 

 various conditions would lead to a better understanding of the matter. 

 If the superficial film act by diffusion or absorption, the transmitted 

 light might be expected to fall off. On the other hand, the mere 

 interposition of a transparent layer of intermediate index would 

 entail as great an increase in the transmitted as falling off in the 

 reflected light. There is evidently room here for much further investi- 

 gation, but I must content myself with making these suggestions. 



Plate Glass Silvered Behind. 

 This glass was silvered chemically by the milk-sugar process, and 

 by transmitted light showed the sky of a normal deep blue colour. 

 The film was not polished. In determining the efficiency of this and 

 other good reflectors, the black glass mirror D' was replaced by one 

 silvered behind. The first trial without a revolving disk gave for the 

 reflecting power 0'82. This result, of course, depended entirely upon 

 the assumed influence of various obliquities of the adjusting plate. A 

 disk was therefore prepared with two opposite projecting teeth, in 

 which the ratio of aperture to circumference turned out on careful 

 measurement to be 0*8230. This number, therefore, represents the 

 transmission of light by the disk. Using this disk I found the follow- 

 ing values for the reflecting power of the mirror for light incident 

 upon it at an angle of 13° 52' : — 



Aug. 11 0*823 



„ 12 0*833 



. Mean 0*828 



This result relates, like all the others, to light polarised in the plane 

 of incidence. Mirrors of this kind are durable, and not being exposed 

 to tarnish are more convenient than specula, whenever the double 

 reflection is not objectionable. The high reflecting power is a satis- 

 factory feature. 



