May 27, 1886] 
NATURE 
OI 
is one additional condition to be fulfilled, viz. that the general 
radius of curvature must be made accurate within very narrow 
limits. In figuring a plane mirror to use, for instance, in front 
of even a small objective, say 4-inch aperture, an error in radius 
which would cause a difference of focus of =4, of an inch would 
seriously injure the performance. This would be about equiva- 
lent to saying that the radius of curvature of the mirror was 
about 8 miles, the versed sine of which, with the 6-inch sphero- 
meter, would be about s¢$y0 Of an inch. Now what I mean 
to convey is this: that it would be just as difficult to figure a 
convex or concave lens of moderate curvature as a flat lens of the 
same size if it were necessary to keep the radius accurate to that 
same limit, 7.2. one-tenth of a division of this spherometer. 
Lick Observatory.—For the final testing of large objectives or 
mirrors, it is necessary to have them properly mounted, and in 
such a manner that they can be directed conveniently on any 
celestial object, and kept so directed by clockwork, to enable 
the observer to devote his whole attention to the testing. I 
had not intended touching at ail on the subject of the mounting 
of telescopes, but I have been asked to call your attention to 
this model of a proposed observatory for Mount Hamilton, 
California, as it embraces some novel features, but I shall do so 
in only a very few words. 
Most here are probably aware that a monster observatory {s 
in course of erection in California, a large sum of money having 
been left for the purpose by a Mr. Lick. The observatory is 
already partly complete, and contains some excellent instru- 
ments of moderate size, the work already done with which 
warrants us to hope that the great 36-inch refractive about to be 
erected will be placed under more fayourable conditions for work 
than any other large telescope in the world. 
The 36-inch objective is at present in process of construction 
by the Messrs. Clark of America, but the mounting has not yet 
been contracted for. 
Some years since, in a paper published in the Zyazsac’ions of 
the Royal Dublin Society, I shadowed forth a principle which I 
considered should be adopted in great telescopes of the future. 
The trustees of the Lick Observatory having invited me to 
design an instrument for the 36-inch objective, { have put into 
practical form what I had before given but general principles of, 
and the design which this model illustrates is the result. 
Whether this design will ever be carried out or not I cannot 
tell, but even as a proposal [ trust it may be interesting enough 
to excuse my introducing it (somewhat irrelevantly perhaps) to 
your notice to-night. 
The design includes the equatorial itself, with its observatory, 
dome, and provision for enabling the observer to reach the eye 
end conveniently. 
The conditions I laid down for myself in designing this obser- 
vatory were that it would be possible for the observer single- 
handed to enter the equatorial room at any time, and that, with- 
out using more physical exertion than is neces-ary for working 
the smallest-sized telescope, or even a table-microscope, he 
should be able to open the 7o-foot dome, turn it round back- 
wards and forwards, point the equatorial to any part of the 
heavens, revolving it in right ascension and declination to any 
extent, and finally (the most difficult of all) to bring his own 
person into a conyenient position for observing. I say this last 
is the most difficult of all, for 1 think any who have worked 
with larger instruments will allow that there is generally far 
more trouble in moving tle observatory chair (so called) and 
placing it in proper position than in pointing the instrument 
itself. In this instrument the ‘‘chair” would require to be 
25 feet high, and with its movable platform, ladder, balance- 
weight, &c., would weigh probably some tons. Even if very 
perfect arrangements were made for the working of this chair, 
the mere fact that the observer, while attempting to make the 
most delicate observations, is perched upon a small and very un- 
protected platform 25 feet above the floor, and in perfect dark- 
ness, tends to reduce his value as an observer to an extent only 
to be appreciated by those who have tried it. 
No matter how enthusiastic a man may be at his work, I 
would not put a high value on his determinations if made while 
in a position which calls for constant anxiety for his own per- 
sonal safety. I would go even further still, and say that even 
personal comforts or discomforts have much to do with the value 
of observations. 
I propose, therefore, that all the various motions should be 
effect'd by water-power. There are water engines of various 
forms now made, some of which have no dead point, and having 
little ws zzertia, are easily stopped and started, and are conse- 
quently well adapted for this work. 
I propose to use four of them: one for the right ascension 
motion of the instrument, and one for the declination ; one for 
revolving the dome, and one for raising and lowering the 
observer himself ; but instead of having anything of the nature 
of a 25-foot chair or scaffold, I propose to make the 70-foot floor 
of the observatory movable. It is balanced by counterpoise 
weights, and raised and lowered at will by the observer. Then 
the observer can without any effort raise and lower the whole 
floor, carrying himself and twenty people if desired, to whatever 
height is most convenient for observation ; and wherever he is 
observing, he is conscious that he has a 70-foot floor to walk 
about on, which even in perfect darkness he can do in sifety. 
The valves and reversing gear of the water engines are actuated 
by a piece of mechanism, the motive power of which may be a 
heavy weight raised into position some time during the previous 
day by man- or water-power. By means of a simple electrical 
contrivance, this piece of machinery itself is under the complete 
control of the observer, in whatever part of the room he may 
be, and he carries with him a commutator of a compact and con- 
venient form, with eight keys in four pairs, each pair giving for- 
ward and backward movements respectively to (A) telescope 
movement in right ascension; (B) telescope movement in 
declination ; (C) revolution of dome ; (D) raising of floor. 
The remaining operation—opening of shutter—is easily 
effected without any additional complication. 
It is only necessary to anchor the shutter (which moves back 
horizontally) to a hook in the wall and move the dome in the 
opposite direction by motion C ; the shutter must, of course, be 
opened by this motion. . . . 
It is very possible that there may be some here who have 
found what I have had to say on the subject of the figuring of 
objectives very unsatisfactory. They may have expected, natu- 
rally enough, that, instead of treating of generalities to such a 
large extent as I have done, I should have given precise direc- 
tions, by the following of which rigidly any person could make 
a telescopic objective. 
To those, however, who have followed me in my remarks, the 
answer to this will probably have already suggested itself. It is 
the same answer which I give to those who visit my works and 
ask what the secrets of the process are, or if I am not afraid 
that visitors will pick up my secrets. All the various processes 
which I have described up to that of the figuring are, I consider, 
purely mechanical processes, the various details of which can be 
communicated or described as any mechanical process can be ; 
but in the last final and most important process of all there is 
something more than this. A person might spend a year 
or two in optical works where large objectives are made, 
and might watch narrowly every action that was taken, see every 
part of the process, take notes, and so forth, and yet he could no 
more expect to figure an objective himself than a person could ex- 
pect to be able to paint a picture because he had been sitting in 
an artist’s studio for the same time watching him at his work. 
Experience, and experience only, can teach any one the art, and 
even then it is only some persons who seem to possess the power 
of acquiring it. 
A well-known and experienced amateur in this work declared 
his conviction that no one could learn the process under nine 
years’ hard work, and I am inclined to think his estimate was 
not an exaggerated one. 
True, it may be said that large objectives can be and are 
generally turned out by machinery, but what kind of an objective 
would any machine turn out if left to guide itself, or left to 
inexperienced hands? 
At the risk of being accused of working by what is generally 
called the rule of thumb, I confess that conditions often arise, 
to meet which I seem to know intuitively what ought to be done, 
what crank to lengthen, what tempering is required of the pitch 
square ; and yet if I were asked I should find it very hard to 
give a reason for my so doing which would even satisfy myself. 
I may safely say that I have never finished any objective over 
10 inches diameter, in the working of which I did not meet with 
some new experience, some new set of conditions which I had 
not met with before, and which had then to be met by special 
and newly-devised arrangements. 
A well-known English astronomer once told me that he con- 
sidered a large objective, when finished, as much a work of art 
as a fine painting. : 
I have myself always looked upon it less as a mechanical 
