420 OPTICAL INSTRUMENTS AND SILVERING GLASS SURFACES. 
had a telescope about 1} inches aperture, and the last about 4 inches ; 
and the results which are given in that same volume of the annals of 
Harward College Observatory are amazing in their accuracy. Jam 
going to make an effort to resuscitate it, because I think it is capable 
of more accurate work than the instruments which are now used for 
practically the same purpose. There is another point about it. That 
is, the symmetry as regards the strain on any part ; when the thing is 
rotated in azimuth, there is not the least additional strain anywhere ; 
you cannot move any other instrument either equatorial or transit circle 
without inducing strain. There is no doubt there is a charm about this 
instrument, from the simplicity of its design, apart from the remark- 
able accuracy of the results obtainable. 
The next instrument I wish to speak about is a modification of the 
equatorial telescope—where instead of having a telescope carried on a 
polar axis in the usual way—we make the polar axis hollow and by a 
plane mirror and the rays from an object glass carried on an arm, up 
the tube to the top to where the observer sits looking down into it. 
This is very like the coudé or bent telescope of M. Loewy, of Paris, but 
there is only one plane mirror, at Cambridge, where they were about 
to mount one of Cooke’s new objectures in the ordinary way, I have, 
with the help of my friend, Professor Turner, of Oxford, induced the 
authorities of the observatory to try this plan so that we shall soon 
have a most important experiment put to the proof with, I hope, 
perfect success. 
The next instrument is one that, I think, is practically one of the 
most important things that we have had in astronomy for a long time, 
that is, the ccelostat of M. Lipman. It is not really Liipman’s, and it 
is not mine or anybody else’s, but August’s, who, in 1840, published an 
account of it. It is a thing that would have schemed itself sooner or 
later, because it is so obvious a thing when you come to consider it. 
It is simply a large plane mirror, mounted parallel to a polar axis 
which is rotated at half the rate of the earth’s rotation, that is, rotated 
once in 48 hours, and it has this property that any part of the sky 
which you can see reflected in it can always be seen, and unlike the 
siderostat, the image does not rotate; so that you can set this mirror 
to any part of the sky within limits ; you can put your telescope on it 
and set the clock going, and the object remains perfectly steady, the 
only thing that is moving is the ccelostat. It is a fixed heavens to all 
intents and purposes. For eclipse work this is an ideal instrument. 
As the sun never departs many degrees from the equator, the conditions 
never become unfavourable geometrically. The observing telescope 
can be fixed firmly, as it need not move for any one declination. In 
the case of solar eclipses, the usual plan has been to take equatorial 
telescopes with their domes or houses which are cumbersome and 
heavy, and in many cases very difficult to transport to the required 
site. The use of the ccelostat will greatly reduce this cost and Jabour. 
We have had four of these new instruments made for observing the 
next total solar eclipse in August next, two of these go to Japan and 
two to Norway, where I hope to go and to have the pleasure of at least 
one officer Royal Artillery as an aid in observing with one of these 
