4.22 OPTICAL INSTRUMENTS AND SILVERING GLASS SURFACES. 
I hope Major McMahon, who has kindly offered to help me as an 
observer also), we may see the thing under much more favourable cir- 
cumstances. We cannot expect that we will beso favoured as those at 
Japan, where the sun will haye an altitude of about 45° and where the 
duration of the eclipse will be a little longer; but it is a long way to 
goto Japan. It is a most delightful thing to think about, even if one 
cannot go, going to Japan and with an eclipse thrown in. Of course, 
a solar eclipse to different minds has a different value, but to anybody 
it must be a most wonderful thing to see, and I should think it was 
well worthy of even such a long journey. But the journey to Norway 
is only such a one as many people take just for mere pleasure just at 
that time of the year. (Applause). 
DISCUSSION. 
Tor CHAIRMAN—By way of starting a discussion I should like to ask Dy. 
Common one or two questions with regard to silvered mirrors. I have to deal 
to a considerable extent in mirrors for range-finders and I find a great deal of 
difference in the light reflected from different mirrors by different makers. 
In the first place the mercurial silvering seems to me to be brighter than the 
deposited silver, I do not know whether Dr. Common can give us any experience 
with regard to that. Then again, I find that the deposited silver is different from 
different makers, that the image is more distinct with some than with others. 
When I have been observing on a dull day I have found a distinct difference be- 
tween two sets of deposited silver glasses ; and the question is, whether the system 
of depositing is answerable for this difference, or has it something to do with the 
want of cleanliness in the glass before the silveris deposited. I suppose that to 
some extent the value of the reflecting surface varies according to the method em- 
ployed for depositing the silver. 
Then again, Dr. Common can perhaps tell us how to test mirrors as to their 
quality of reflection—whether there is any way of comparing one mirror with 
another. We often find a difficulty when inspecting mirrors to say whether they 
are up to the standard in quality, as so much depends on the state of the atmos- 
phere at the time of testing. With regard to the ccelostat, would not the fact of 
having to observe by the aid of a large mirror of large surface, which it must be 
difficult to obtain perfectly true, rather detract from the value of that instrument. 
Proressor C. V. Boys, F'.R.S. (Applause) —Mi. Chairman and Gentlemen, when 
T heard that Dr. Common was going to lecture to you upon the subject of the 
silvering of mirrors and the care of optical instruments I was exceedingly anxious to 
be able to have leisure to come and hear him, because I, myself, have for many years 
been but an amateur in matters astronomical and have experienced some of the 
difficulties which we have had examples of this afternoon, of silvering mirrors 
satisfactorily. I remember not very long ago, in conversation with Dr. Common 
on this very subject of silvering mirrors, that he incidentally mentioned the use 
of the protochloride of tin. It was not generally known at that time ; it certainly 
is not given in any text-book that I have read; and I need hardly say it was 
absolutely new tome. In consequence, directly I got back to Kensington I caused 
some mirrors that were going to be silvered to be splashed—not properly dipped 
in but just splashed with a little protochloride of tin solution—and to be care- 
lessly washed off; the effect was most marked and beautiful; every little mark of 
the chloride of tin solution, every place on which it had been splashed shewed 
itself at once in the silvering, producing a denser, thicker and more uniform de- 
posit, And now in consequence of this information obtained in this accidental 
