622 
NATURE 
[AUGUST 15, 1912 
cross of narrow and shallow channels. Such channels | for a minute or two; if a hill is suspected, he washes 
are found to be necessary in order to secure an even 
distribution of the emery and water all over the sur- 
face. The grooved tool is now used for many hours, 
and the surface is tested at frequent intervals with a 
spherometer. The work ceases when it is no longer 
possible to detect errors of curvature in this way. 
The next stage is polishing. The thickness of the 
layer of glass worn off in polishing is to be estimated 
in ten-thousandths of an inch, and can scarcely be 
detected even with the finest spherometer. For polish- 
ing the iron tool is discarded and the work is carried 
on by hand. As lightness is essential, the tool is 
built up by a stiff lattice-work of wood with a con- 
tinuous wooden working face. It is obvious that 
however carefully the face may be turned it cannot 
be made sufficiently true, and the requisite accuracy 
is obtained by means of the plastic properties of rosin 
or pitch. A number of squares of rosin about 
a quarter of an inch thick and an _ inch 
square are made, and these are glued in rows 
on the convex face of the wooden tool, with a 
narrow space intervening between each rosin square 
and its neighbours. The tool is then warmed slightly 
so as to soften the rosin a little, and it is then pressed 
lightly on to the glass disk. By means of this ‘“‘ warm- 
pressing’ a nearly perfect fit is attained. 
Each of the rosin squares is then painted with hot 
melted wax. This is done because wax is harder than 
rosin and affords a better working face. Finally, 
when the tool is quite cold, the surface of the glass 
is painted all over with very finely powdered rouge 
and water, and the tool is placed gently on the glass 
with some additional weight resting on it. It is left 
thus for several hours, but is moved slightly every 
ten minutes to ensure an even distribution of the 
rouge and water. By means of this ‘‘cold-pressing ”’ 
a perfect fit is secured of the wax-coated rosin squares 
with the glass face. Cold-pressing has to be repeated 
every day before the work begins. 
The polishing is now carried on in much the same 
way as the grinding, but by hand instead of by 
machine power. The turn-table can be made to tilt 
so as to bring the glass to stand vertically, instead 
of horizontally, and the disk is frequently tilted up 
so as to submit the surface to optical tests. These 
latter tests are far more searching than those with 
a spherometer, and enable the observer to detect an 
error in the radius of curvature of portion of the 
reflector of a hundredth of an inch. To correct such 
an error it will be necessary to remove a layer of 
glass of <jqoonth of an inch} 
The most refined optical test is by the observation 
of the image of a brilliant light issuing from a pin- 
hole close to the intended centre of the spherical 
surface. The observer examines the image of the pin- 
hole with a microscopic eyepiece placed as close as 
possible to the pinhole. He then causes a straight- 
edge close in front of the eyepiece to move slowly 
across the reflected beam of light, either from left 
to right or from right to left, so as to eclipse the light. 
Previously to the eclipse the whole of the glass seems 
to be a uniform blaze of light, and if the curvature is 
perfect the light which enters the observer’s eye comes 
from all parts of the disk, and the surface is seen 
to darken equably all over. But if the surface is 
imperfect the light from some part is eclipsed sooner 
than that from others, and the disk seems to possess 
considerable hills and valleys illuminated, as it were, 
by a setting sun. 
The interpretation of these apparent hills and valleys 
shows where further local polishing with a small tool 
is requisite. Sir Howard Grubb says that if he sus- 
pects a hollow, he holds his hand near the surface 
NO. 2233, VOL. 89| 
the region with an evaporating wash. The warmth 
in the one case and the cooling in the other tend 
to rectify, and indeed over-rectify, the errors. 
When success is finally attained, after all we have 
only a spherical surface, and it becomes necessary to 
obtain a parabolic form. This last stage is done by 
further tests of the kind described, with a diaphragm 
placed over the mirror which only permits the 
observer to see the light reflected from chosen zones 
of the mirror. The time at my disposal will not allow 
me to describe this in further detail, or to tell you 
how there is always found to be one definite diameter 
of the glass along which its weight must be sup- 
ported. JI must pass by, too, the system of counter- 
poised levels used for supporting the back of the 
glass, and the method by which silver is chemically 
deposited on its surface. Meagre although this sketch 
has been, it will have served to show you how beau- 
tiful are the processes employed, and I would ask 
you to realise that at first Herschel was a mere 
amateur, and had to discover everything for himself. 
As I have said, Herschel had to do all his polishing 
by hand, and he found when once the final stage 
had begun, it was necessary that it should never stop 
even for a moment. Caroline relates how she was 
kept busy in attending on her brother when polish- 
ing: ‘‘Since by way of keeping him alive I was 
constantly obliged to feed him by putting the victuals 
by bits into his mouth. This was once the case, 
when in order to finish a 7-ft. mirror, he had not 
taken his hand from it for sixteen hours together.” 
The making of the mirror is, however, but a small 
part of the difficulty of making a telescope, for it 
involves high engineering skill to provide a_ solid 
stand, an observing platform, the graduated circles 
in right ascension and declination for setting the 
telescope and the clock, whereby it is made to follow 
the stars in their daily motion. The great size of 
Herschel’s mirrors and the weight of the long tube 
introduced mechanical difficulties which were at that 
time entirely new. 
A dozen years after his establishment at Bath, 
Herschel began to be well known in the world of 
science, and many of the most illustrious astronomers 
came to see him. In 1781 he was elected to the Royal 
Society, and in the same year he discovered the planet 
Uranus, and called it by the now almost forgotten 
name of Georgium Sidus, in honour of George III. 
The magnitude of the discovery may be estimated 
by the fact that only the five principal planets, fami- 
liar to all men for centuries, were then known; and 
the asteroids or minor planets had not yet been dis- 
covered bv Herschel himself. His fame from this and 
his other discoveries led to a command from the King 
to take his 7-ft. telescope to Windsor, and there he 
was requested to act as celestial showman to the 
King, the Queen, and the Princesses. The expedition 
put him to much expense, and he was kept hanging 
about Windsor for months, but at length the King 
offered him the post of Private Royal Astronomer, 
with the modest salary of 2001. a year. 
Herschel’s friend, Sir William Watson, said that 
never had a monarch bought honour so cheap, and 
Caroline pours scorn on the King’s meanness; but I 
think this was scarcely fair. It must have been well 
known that Herschel had deserted from the Hano- 
verian Guards, and while the King might consent 
to forget this, it was a strong measure to take the 
deserter into his service. At a later date, moreover, 
when the King was informed bv Sir Joseph Banks of 
Herschel’s financial difficulties, he granted him 2o00l., 
afterwards increased to 4oool., for the construction 
of the great 4o-ft. telescope, with the condition that 
