OPTICAL INSTRUMENTS AND SILVERING GLASS SURFAGES, 423 
way the use of protochloride of tin is, at our place at any rate, universal ; it ought 
to be universal everywhere, because it facilitates the process so enormously. I 
wish Dr, Common could explain it by some more satisfactory term than that of 
catalytic action, a term which is generally used to hide ones defective knowledge. 
With respect to that very beautiful instrument of Dr. Chandler’s, I must say Twas 
quite as surprised as I ought to have been when I heard of the extraordinary 
precision with which such a telescope would point, when removed from any 
particular position, round a vertical axis and brought back again; certainly one 
would not have expected that a telescope would recover its position with an 
accuracy which corresponds to the optical defining power of an instrument even 
so small as two inches aperture or thereabouts. 
Dr. Common was good enough to put to mea question that I could not pro- 
perly catch, because owing to the brilliance of the light and the reflective power 
of the blackboard (which is something like a silvered mirror, being dark and 
shiny) I was quite unable to see a single mark that he made uponit. I do not know 
whether the marks are there now, but I cannot answer the question because I do 
not know what it was. 
Dr. Common—They are rubbed off now. 
Prorgessor Boys—I should also like to know exactly what the observation is 
that is made. Is it taking the time at which a star passes this circle on the east 
and then passes this circle on the west ? 
Dr. Common—Yes. 
Prorgessor Boys—Very well. With regards to the coelostat that again is an 
instrument which was certainly new to me when I read about it last year I think. 
The instrument is one that I should like to have had very much when about six 
or seven years ago I was experimenting on the heat radiated by the stars ; I be- 
lieve in lecturing on quartz fibres in this Institution a year or two back, I gaye some 
short account of those experiments. The difficulty there was to mount a large 
telescope—I had a 16-inch mirror lent me by Dr. Huggins—so as always to bring 
the light from the star to one point in space, and always to bring it horizontally 
to that point, because that was the point at which the sensitive surface of the 
radio-micrometer was placed, and the instrument is of such a kind that it cannot be 
pointed about as it would have to be if it were attached to an equatorial telescope. 
So I designed a special instrument, and again I took advantage of Dr. Common’s 
great practical knowledge in submitting my designs to him before they were sent 
to the engineer to be executed, and I got some very valuable suggestions. Among 
others I remember perfectly well the exact words he used—I made a drawing of 
the suggested instrument and was a little alarmed as to whether the iron-founder 
would succeed in making the somewhat complicated form; I did not quite see 
how to cast it myself; and he comforted me by saying, “ They can cast anything.” 
Well, they did cast it all right, and I did not make a more elaborate pattern than 
I should naturally have done. This ccelostat would not haye had, for my purposes, 
the particular advantages that it possesses from the point of view of eclipse ob- 
servation or photography of a portion of the sky, namely, that it not only brings 
some particular star into a definite position as seen in the telescope and keeps it 
there quiet, so that it or its spectrum may be quietly examined, but it keeps that 
star and the neighbouring stars in the same relative position. In the heliostat 
you see the star or the sunall right in the telescope, but as time goes on the object 
turns round about the axis of the telescope as acentre. If the object is to be photo- 
graphed, especially if the exposure is long, the ccelostat is necessary, as the image 
is prevented from turning round, you see it stationary so that two stars oneabove 
the other will always remain so, and not turn head over heels as they would do if 
seen with a heliostat. The remarks that your chairman has made on the possible 
objection owing to the fact that there is another surface, are exceedingly pertinent, 
