418 OPTIGAL INSTRUMENTS AND S'LVERING GLASS SURFACES. 
only has that been an immense improvement to the telescope itself but 
it has enabled us to use reflecting surfaces in other ways. For instance, 
we have now the equatorial coudé and the ccelostat—about which I 
wish to speak more particularly later on—both depending for their 
work upon accurate and strongly reflective plane surfaces; therefore 
to the astronomer particularly, the best silvered surface is a very 
important thing, and as it is quite an empirical process, I will show 
you the various results that are obtained by different processes. 
Liebig was the first to find out that aldehyde has the power of 
depositing the metallic silver from the oxide; and other men have 
worked on the same subject with more or less success until we have 
now got two or three excellent processes applicable to different ways 
of depositing the silver, hot or cold, upwards or downwards, which are 
sufficiently easy and practicable to be used by almost anyone. The 
first experiment I will show you will be the simple one of depositing 
silver from an ammoniated solution of the nitrate with tartaric acid ; 
that, I think, is the simplest process that I know of. The next will 
be depositing silver from a solution of ammonia nitrate, using Rochelle 
salts. The next will be the sugar process, using not only ammonia 
nitrate but a mixture with potash in it, by means of which in the up- 
ward process of precipitation you get a very beautiful film. It will 
not take long to prepare these things, and while they are working out 
their duty I can speak of the optical instruments. The lecturer then 
proceeded to explain the different processes of silvering glass, dwelling 
particularly on the several points that require attention during the 
operation, such as the thorough cleaning of the surface, the use of a 
weak solution of protochloride of tin as a wash for the surface of the 
glass immediately before using the solution and the great importance 
of the sherry colour of the silver solution before the addition of the 
reducing agent—a practical demonstration of the various processes was 
given by silvering various glass vessels. 
Now while these things are doing their duty in the best way they 
can—lI do not know whether they will behave properly or not—I would 
just like to say a few words upon the other branch of the lecture. I 
have had occasion lately to read up the papers of an American 
astronomer to whom we have given the gold medal of the Royal 
Astronomical Society, Dr. Chandler. He some 12 or 14 years ago 
invented an instrument which he called the almucanter, which involves 
such an absolute departure from preconceived notions and has in its 
use proved such a remarkably good instrument that it is worthy of, you 
might almost call it, resuscitation, for it has been buried for many 
years now—nobody has ever made another that I have ever heard of, 
although the results which are given in the 17th volume of the annals 
of the Harvard College Observatory show its remarkably and wonder- 
fully accurate measurements. In illustration of something else here, I 
have brought a little model—it is not a very complicated thing, but it 
is sufficient for the purpose. If we take this model as the polar axis 
that astronomical instruments like the equatorial are mounted on, and 
which by one rotation will carry any instrument fixed upon it round so 
as to follow the stars by one motion, we have an example of the 
