AND A SELECTION EEOM THE OBSEEVATIONS MADE WITH THEM. 695 
matter resting upon the iron will act just as a chip of wood. If the lubricating fluid 
is a little in excess the rouge will run loose, the very hard resinous surface being able 
to retain but a very small quantity of it, and the incipient polish will disappear. An 
excess of rouge acts in the same way, while, if the rouge is not in sufficient quantity to 
keep up the cutting-action, the surface of the speculum loses its truth. The process 
therefore requires great attention throughout. Both the temperature of the water in 
which the speculum revolves and the temperature of the room, of course, must be pro- 
perly regulated. The process does not proceed well unless the moisture between the 
speculum and polisher gently evaporates, so that drops of fresh fluid may be added from 
time to time, to carry away the undue collection of abraded matter. As the hygro- 
metric state of the air varies, so Avill the quantity of fluid required to lubricate the surface ; 
and that would be a source of considerable embarrassment, were it not that in dry states 
of the air the dew-point can be adjusted by a jet of steam. When the air is very damp 
we have no practical remedy ; and therefore the operation is not then attempted. 
We have often endeavom-ed to evade these difficulties by employing a surface less 
hard, supported by a thicker substratum of pitch ; but there has been an evident sacrifice 
of ttruh of surface and figure, and we have failed in ob taming that very fine definition 
which resulted from the old process when perfectly successful. By the old process, a 
speculum of 3-feet aperture and 27-feet focus has been frequently made so perfect that 
in favourable states of the ah it has defined sharply the dots and figures on a watch- 
dial distant 100 feet, the eye-glass being a single lens of one-eighth of an inch focus; 
such a speculum in ordinary weather perhaps does little more than one that is inferior 
to it, both, for instance, showing w'ell the sixth star in the trapezium of Orion ; but in 
extremely fine nights it displays its powers by resolving nebulae in which no traces of 
resolution had been seen before, and by concentrating the light of minute stars and so 
rendering them visible. 
If the vivid polish of a speculum employed in the 0])en air was as enduring as that of 
glass, the difficulty of the process and its uncertainty without continual practice would 
have been no great objection to it; but when, on the contrary, it is necessary to repeat 
the process at intervals perhaps so long that minute details are not fresh in the memory, 
the task becomes the labour of Sisyphus. 
A very fine speculum loses much of its light and some of its truth of surface by 
being repeatedly dewed, especially if it has been several times cleaned, and for the 
ordinary work we are engaged in will be inferior to a moderately good speculum which 
is quite fresh. 
The preparation of a polisher in the way formerly described is one of the great diffi- 
culties ; a certain degree of manual dexterity is required, which can only be obtained 
by practice and kept up by practice. For many years we have very often prepared it in 
an easier way : some pitch of the proper consistence for polishing at 55° is put into 
warm water ; and when soft, a little is taken out and rolled upon a wet board to the 
proper thickness. There is no difficulty in this, as the roller is governed by ledges of 
MDCCCLXI. 5 B 
