424 OPTICAL INSTRUMENTS AND SILVERING GLASS SURFACES. 
because the difficulty of producing a large plane surface which must be plane in 
an extraordinary degree, even more perfect as a plane than the mirror is as a 
parabola—the difficulty of producing such a surface is exceedingly great. It is 
true that Dr. Common is himself a master in the art of producing spherical para- 
bolic and plane surfaces, and perhaps he has not thought it worth while to mention 
that it is not so easy a thing to make a truly plane surface; in fact until Lord 
Rayleigh a year or two ago invented a new process there was no method by which one 
could with extreme accuracy test not only whether a thing was a plane but see 
exactly how much it was in error in every part using a quarter wave of light or the one 
hundred and sixty thousandths of an inch as the unit of observation. Of course 
now one cannot explain the process invented by Lord Rayleigh, but I will simply 
say that Lord Rayleigh has done for optical workers working on plane reflecting 
surfaces exactly what Sir Joseph Whitworth did for mechanical engineers. Before 
Sir Joseph Whitworth’s time pieces of iron were ground and rubbed together until 
it was imagined that they would both be flat, which neither of them could be; his 
process of scraping and the creation of surface plates has made a revolution in the 
possibilities of mechanical engineering. And I believe inthe same way the pro- 
cess of Lord Rayleigh will do precisely the same thing where not merely com- 
paratively small surfaces are concerned, but where these very large plane mirrors 
which will be required when any such method as this is used are necessary in 
order to enable a large telescope to be used. I feel that if these large mirrors can 
be made fairly easily and fairly cheaply and still with this enormous degree of 
accuracy which is essential, then there will be a great advantage from instruments 
of this sort. Had I been able to get any such mirrors when I was making my 
experiments on the heat of stars I should have been only too glad to have been 
able to keep my telescope quiet and still in a horizontal position, instead of hay- 
ing to employ a telescope wandering about in all directions and get so mounted 
that wherever it was pointing it would bring the light from the star horizontally 
to one fixed point. (Applause). 
Caprain M. B. Luoyp, R.A.—There is one point I should like to ask Dr. 
Common about, and that is with regard to silver reflecting surfaces. The reflected 
light from silver is slightly red in colour, and | wouldask whether that makes any 
difference with regard to spectroscopic action and stellar photography. Palladium 
gives a very white reflection, and ] want to know whether it has been applied 
at all to mirror silvering or mirror plating. 
REPLY. 
Dr. Common—wWith regard to the question put by the last speaker, I have 
made no experiments on the reflecting power of palladium, but I know that it can 
be deposited, and possibly it may be a very useful metal to use. Of course any- 
thing deposited on a mirror which is of the shape of the ultimate metal assumed 
can be taken off and tried again. ‘The form is given by the glass mirror and the 
film which is then deposited converts that glass mirror into a metal one—the same 
metal as the film. So that with a good glass surface it is possible to try all these 
yarious experiments and determine absolutely the effect. With regard to ab- 
sorption of the blue rays there is no doubt a slight absorption by the silver; but 
considering the efficacy of silver on glass telescopes in phographing the most 
delicate objects in the heavens (that is the nebulae) we cannot say that it is very 
deficient in actinic power. In fact, up to now it. has exceeded the achromatic ; 
but for some reasons it does not give the same delicate points with stars ; there 1s 
more diffusion of light in the reflecting than in the refracting. I have no doubt 
that it may be quite possible that we may get a metal which will give us a whiter 
image than silver. I have noticed the red colour in using speculum metal, but I 
have never noticed that with the silver on glass. 
