298 
Construction of Object-glasses. 
["Monthly Mien scopical 
L Journal, May 1, 1»69. 
The most convenient way for the amateur of reducing the sub- 
stance, or giving the rough rounded form to small lenses, is a large 
plate of zinc and coarse emery and water; iron is too hard, lead 
too soft, and copper poisonous. 
Of the Powders emjployed for Grinding and Polishing Glass. 
For lenses, emery is almost invariably employed for rough 
grinding and smoothing. For the latter operation, it must be 
washed to various degrees of fineness, as it is seldom sold in this 
state, the sizes in commerce are merely sifted. Emery differs much 
in hardness and quality, according to the locality from which the 
ore is obtained. If it is full of small reddish particles of a dull 
slaty appearance, it is soft and deficient in the grinding property^ 
The Guernsey emery is of this character and very inferior to the 
Naxos, the particles of which have a steely appearance of uniform 
colour ; but this latter is difiicult to obtain, as it is monopolized by 
some of the large plate-glass manufacturers. Three or four sizes 
are sufiicient for the glass-worker for roughing down and fine grind- 
ing ; but for smoothing, washed emery of several degrees of fineness 
are required. A portion of the flour of emery of commerce is 
placed in a bowl, or a common washhand basm, and well stirred up. 
At the end of ten seconds the water is poured into another bowl ; 
this is repeated several times, till no more can be withheld from the 
original quantity. This washed quantity is again separated into 
several other degrees of fineness, as at the end of one minute, five, 
twenty, and sixty minutes ; but after one hour, a very small quantity 
is obtained from one pound of the flour of commerce. This being 
of value for the perfection of the final smoothing, or obtaining a 
semi-polish on the metal lap or mould itself, I have preferred pro- 
curing it from the "optician's mud," or refuse of the previous 
grinding operations. Taken in an unprepared state, this contains 
a large percentage of impurities, consistmg of ground-glass and 
metal particles from the laps ; it is therefore necessary to remove 
them. The first by boiling the mud with caustic potash, and after 
washing away all trace of the alkali, finally treating with dilute 
sulphuric acid. The finest portion only of one hour's suspension 
may then be separated and obtained in a satisfactory quantity. 
The polishing powders used by the workers of minute lenses, 
are putty-powder, or oxide of tin, and crocus, or peroxide of iron. 
The first may be obtained sufficiently good without any difficulty ; 
but after many trials both by roasting the alkaline precipitate from 
sulphate of iron, and also carefully washing the crocus of commerce, 
have given the preference to jewellers' rouge, sold by Acton, of 
Farringdon Street. In this form it is far too soft for glass polish- 
