PHYSICS, CHEMISTRY AND ENGINEERING 189 



of a spectrometer, thus facilitating the determination of the 

 angles of incidence of the light on the alkali mirrors. 



Great difficulties were encountered in making the mirrors. 

 These were made both by repeated distillations and pourings. 

 The metal was deposited on a glass plate which formed a part 

 of a small cell. The reflecting power of the alkali metals in 

 contact with the glass is given by R= t'd-i-Or'-r-r') 



O is the fraction of incident light reflected by the whole 

 mirror, (metal plus glass) ; r is the reflecting power of the 

 glass surface from air to glass to air; r is the reflecting power 

 of the glass surface going from the glass to air to glass ; and t 

 is the transmission power of the glass plate for a single passage 

 through the glass plate. 



The values of t and r' for various angles of incidence are 



, ., .- . T 2 4-(l-R')(R'-r). ♦_ (R'-D(l-r) 



given by the equations, t— ^ T(1 . r) ; r — T , + (1 _ R>) (R .. r) 



T is the fraction of incident light transmitted by the glass 

 plate and incident on the photo-electric cell. R' is the re- 

 flecting power of the glass plate (both surfaces). There are 

 no approximations in the derivation of these equations. 



The investigations of the properties of the glass plates, 

 showed that x* is somewhat less than r. The values of r were 

 obtained by abrading and blackening the surface of the glass 

 plate, thus leaving only one surface effective. 



Potassium gave reflecting powers of 88 per cent at 9° inci- 

 dence to 89 per cent at 35°. A mirror formed by pouring the 

 metal against the glass plate gave results coincident with those 

 from one formed by distillation. Only one rubidium mirror 

 has been investigated so far, its reflecting power at an angle 

 of incidence of 9° being 76.3 per cent. This was increased 

 to 78 per cent at 35°. Sodium gave reflecting powers in- 

 creasing from 89 per cent at 9° to 91 per cent at 35°. The 

 reflecting powers of the alkali metals therefore decrease as 

 the atomic weight increases. 



Note — So far, non-polarized light has been used. Subsequent to the report on 

 this work the investigation was carried out, using monochromatic and polarized 

 light. A full account of this investigation is to appear in a fall (1916) number, 

 probably the October number) of the Astrophysical Journal. 



