AUGUST I5, 1912| 
be tempted to underrate the value of educational 
advantages. Concerning education, Bishop Creigh- 
ton once said in my hearing, ‘‘It is surprising how 
little harm we do notwithstanding all the pains we 
talke.’’ Paraphrasing the remark, although spoiling 
the epigram, I would say, “It is surprising how little 
harm the lack of opportunity does to a great genius.” 
In 1766 William took a position as organist at 
Bath, then at the height of fashion. The orchestra at 
the Pump Rooms and at the theatre at Bath was 
then one of the best in the kingdom, and Elizabeth 
Linley, daughter of the director of the orchestra, was 
the prima donna of the concerts. When in 1771 she 
became engaged to Charles Sheridan, Herschel 
thought that the expected vacancy would make an 
opening for his sister at Bath, and suggested that she 
should join him. And, in fact, after a time such a 
vacancy did occur, for Elizabeth Linley, after flirting 
with Charles Sheridan, jilted him, and eloped with 
and married the celebrated Richard Brinsley Sheridan. 
Caroline was very anxious to accede to her 
brother’s suggestion, but the rest of the family would 
not for a time hear of it. At length, however, in 1772, 
Herschel came to Hanover and carried off his sister 
with the mother’s reluctant consent. Even from boy- 
hood his intense love of astronomy had been manifest, 
and it is interesting to note that in passing through 
London on their way from Harwich to Bath, when 
they went out to see the town, the only sights which 
attracted their attention were the opticians’ shops. 
On Mr. Linley’s retirement from the orchestra at 
Bath, Herschel became the director and the leading 
music-master in the town, and he thus obtained an 
established position. Although Caroline sang a little 
in public, her aspiration to become the prima donna 
of Bath was not fulfilled. But she was kept busy 
enough at first in the cares of housekeeping, with 
endless wrangling with a succession of incompetent 
slaveys, and then she gradually became more and 
more her brother’s astronomical assistant. 
In the midst of Herschel’s busy musical life he 
devoted every spare moment to astronomy, and when 
his negotiations for the purchase of a small reflecting 
telescope failed—and they were all small in those days 
—-he set to work to make mirrors for himself. 
One room in the house was kept tidy for pupils, 
and the rest of the house, including the bedrooms, 
was a litter of lathes and polishing apparatus. He 
made reflecting telescopes not only for his own use, 
but also for sale, for the purpose of providing funds 
to enable him to continue his researches. His in- 
dustry must have been superhuman, for later in his 
life he records that he had made more than 400 
mirrors for Newtonian telescopes, besides others of the 
Gregorian type. These mirrors ranged in diameter 
from a few inches to 4 ft., in the case of the great 
4o-ft. telescope. I should say that mirrors are not 
specified by the diameter of the reflecting surface, 
but by the focal length. Thus, whatever may be the 
diameter of the reflecting surface, a 2o0-ft. telescope 
means that the mirror is approximately portion of: a 
sphere of 4o ft. in radius, and this will give a focal 
length of 20 ft. You must, in fact, double the focal 
length of a telescope to find the radius of the sphere 
of which it forms a small part. 
In order to learn anything of the making of re- 
flectors it is necessary to go to original memoirs * 
on the subject, and even of them there are not many. 
I feel, therefore, that I shall not be speaking on a 
topic known to many of the audience if I make a 
digression on a singularly fascinating art. Mirrors 
2 Sir Howard Grubb’s lecture at the R.I. in 1887 is one of these, vol. xi., 
p. 413. Lord Rosse’s papers are amongst the most important. 
NO. 2233, VOL. 89| 
NATURE 
621 
are now made of glass with a reflecting surface of 
chemically deposited silver; formerly they were made 
of speculum metal, an alloy of copper and tin. Of 
whatever substance the mirror is made the process 
of working it to the required form is much the same. 
The most complete account of the process of which | 
know is contained in a paper by Prof. G. W. Ritchey 
in vol. xxxiv. (1904) of the Smithsonian Contributions 
to Knowledge. He there gives a full description of the 
great reflector of the Yerkes Observatory. The process 
only differs from that employed by Herschel in that he 
worked by hand, whereas machinery is now required. 
to manipulate the heavy weight of the tools. The 
Yerkes mirror is formed of a glass disk 5 ft. in 
diameter, and it weighs a ton; the grinding tools are 
also very heavy. ; 
I must pass over the preliminary operations whereby 
the rough disk of St. Gobain glass was reduced to a 
true cylindrical form, smooth on both faces and round 
at the edge. Nor will I describe the grinding of a 
shallow depression on one of the faces by means of a 
leaden tool and coarse emery powder. 
It will be well to begin by an account of the manu- 
facture of the tools wherewith the finer grinding and 
polishing is effected, and then I shall pass on to a 
short description of the way they are used. 
Two blocks of iron are cast with the desired radius 
of curvature, the one being concave and the. other 
convex. The castings are then turned so that the 
concavity and convexity fit together as nearly as 
may be. For the large mirror these blocks are a 
little more than 2 ft. 6 in. in diameter, but for small 
ones they are made of the same diameter as the mirror 
to be ground. The two are then ground together for 
a long time with emery powder and water until every 
part of one surface fits truly to every part of the other. 
They must then both be portions of a sphere of the 
same radius, because the sphere is the only surface in 
which a universal fit is possible. The concave iron 
is very precious, because it furnishes the standard for 
regrinding the convex grinding tools when they have 
become worn by use. In order to make a plane 
mirror, three surfaces are ground two and two, for if 
A fits B and C, and B fits C all over each surface 
they must all be true planes. However, I shall only 
speak of the figuring of concave mirrors. 
The roughly hollowed glass disk is now laid on 
several layers of Brussels carpet centrally on a mas- 
sive horizontal turn-table. The convex iron tool just 
described is suspended by a universal joint from a 
lever, and it is counterpoised so that only a-portion 
of the weight of the tool will rest on the glass when 
it-is in use. A complicated system of cranks and 
levers is so arranged that the tool can be driven by 
machinery to describe loops or curves of any arbitrarily 
chosen size over: the glass, and as these loops are 
described by the tool the turn-table turns round slowly. 
In this way every part of the tool is brought into 
contact with every part of the glass disk in a. sys- 
tematic way.- When working near the edge a large 
part of the tool projects beyond the edge of the glass. 
Emery powder and water are supplied in a way I 
need not describe, and the tool is lowered gently on 
to the glass. The motive power is then applied, and 
the grinding is continued for many hours until the 
preliminary rough depression has been hollowed to 
nearly the desired shape—namely, that of the standard 
concave iron. 
For finer grinding a change of procedure is now 
adopted, and very finely powdered emery is used. 
Another convex tool is formed, by grinding with the 
standard concavity; the working face of the tool. is, 
however, now cut up into small squares by a criss- 
