OPTICAL INSTRUMENTS AND SILVERING GLASS SURFACES. 425 
That brings me to a point which your chairman brought forward, and that is 
the reflective power of the various films. There is no doubt an enormous differ- 
ence ; it is a matter that I have been working at—I was working at it many years 
ago—and for the purpose of determining the comparative reflective power, es- 
pecially photographically, I had some sort of arrangement like this—I must put 
it so that Professor Boys can see it, and I daresay somebody else will too. I had 
up kere a point of light to focus the object glass, and there would be, about 
parallel, a beam of light (describing the same on the blackboard). There was the 
wall of my dark photographing room; ¢his was the beam of light that would 
practically go on with very slight variations, because the hole was a very minute 
one. I took two pencils of that light from equal distances of the object glass and 
that made two square pencils of light that I could do with as I liked. I put on 
various pieces of glass silvered in various ways and had the reflection at different 
angles. I could send on to the photographic plate the two beams from tho dif- 
ferent reflectors, and I found there was an enormous difference; there was a 
difference after the silver had been exposed to the air and when it was first 
polished, and there was a difference between speculum metal and silver. It was a 
very interesting series of experiments that I made, but I did not conclude them ; 
they were in the hands of my assistant at the time and I left the matter to him 
principally and he did not complete them. We would find that there was an 
enormous difference, which we could make quite evident, either with the length of 
exposure to get the comparable images or with the difference of density when the 
same exposure was given to two different reflecting surfaces; it would at any time 
be easy to substitute a photometer for the photographie plate and thus determine 
the loss of light for visual work. 
Tur Cuatrman—lt is the slight differences that are so difficult to detect. 
Dr. Common—You must magnify them. ‘The eye density and the photo- 
graphic density are comparable I believe. I find with the source of light of 
different colours the density varies in fair proportion with the light as seen, and I 
think you would get some sort of a guide at anyrate. 
With regard to the ccelostat and the image there is only one surface, and the 
coudé telescope of M. Loewy has two surfaces; that answers very well indeed. 
When some years ago I saw through it the images were very fine; I have never 
seen through the larger coudé lately finished, but I have seen a photograph which 
shows the details on the surface of the moon and indicates that the definition is 
actually superb. There we have two surfaces and the length of the reflective ray 
is very long, so that the linear dispersion from an irregular surface would be very 
large, and we may considerjthe plane mirror question at an end. No doubt 
when the coudé was first made everyone thought that the plane mirrors would be 
no difficulty, but the brothers Henry of Paris are capable opticians and make 
beautiful flats. Ihave one of theirs which they gave me before I took to flat 
working myself, and I am now fortunately just in a position to criticise their 
work and no more. 
With regard to what Professor Boys said about the protochloride of tin, I am 
very glad to have his testimony to the efficacy of the tin solution. It is one of 
those little things which help to make a person successful, it was told me by a 
gentlemen who had I think to pay for the information, but I had already had a 
search made in the Patent Office and found this mentioned, so I have no scruple 
in mentioning it again. 
Tur CuarrmMan—Gentlemen, I have nothing further to add but to express our 
thanks to Dr. Common for his very interesting and instructive lecture. I hope it 
will not be the last that he will give us, and I may suggest that the care and 
preservation of optical instruments would be a very interesting subject for another 
discourse. (Applause). 
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