46 
THE YOUNa SCIENTIST. 
seen in front. The eyeball forms an 
ellipse, with the sight always in the 
centre. 
The ear, in width, is equal to one eye, 
in length to two. The hand is the same 
length as the face, in width one-half. 
The foot, in profile, is nine eyes in length 
and three in height. 
The proportionate height of a man is ten 
faces; and extending the arms horizon- 
tally their full length the same proportion 
is obtained. 
These measurements are only suggestive 
of some of the more important proportions 
of the human body which the student of 
art and truth must master. 
♦ . 
Home-Made Telescopes and Micro- 
scopes— IV. 
THE POWDERS EMPLOYED FOR GRINDING AND 
POLISHING GLASS. 
SUCCESS in producing well-finished 
lenses will depend a good deal upon 
the excellence of the grinding and polish- 
ing powders used. In regard to this point 
Mr. Wenham gives the following direc- 
tions : 
" For lenses, emery is almost invariably 
employed for rough grinding and smooth- 
ing. 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 dif- 
fers much in hardness and quality, ac- 
cording 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 pro- 
perty. The Guernsey emery is of this 
character, and very inferior to the Naxos, 
the particles of which have a steely ap- 
pearance of uniform color ; but this latter 
is difficult to obtain, as it is monopolised 
by some of the large plate-glass manu- 
facturers. Three or four sizes are suf- 
ficient for the glass worker for roughing 
down and fine grinding ; but for smooth- 
ing, washed emery of several degrees of 
fineness are required. A portion of the 
flour of emery of coinmerce is placed in a 
bowl, or a common wash-hand basin, 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 quan- 
tity. This washed quantity is again separ- 
ated into several other degrees of fine- 
ness, 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 fiour 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 procuring it from the " op- 
tician's mud," or refuse of the previous 
grinding operations. Taken in an un- 
prepared state, this contains a large per- 
centage of impurities, consisting 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 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 ob- 
tained sufficiently good without any dif- 
ficulty; but after many trials, both by 
roasting the alkaline precipitate from 
sulphate of iron, and also carefully wash- 
ing the crocus of commerce, I have given 
the preference to jeweller's rouge. In this 
form it is far too soft for glass polishing ; 
it must, therefore, be heated in an iron 
pot, and dilligently stirred till the mass 
acquires a purple color ; it is then of the 
requisite degree of hardness. Both this 
and the putty-powder must be washed, 
to separate gritty particles ; about five 
minutes will be sufficient. After obtain- 
ing all that can be suspended in this time, 
the residue may be levigated on an iron 
plate, with a soft iron spatula, and the 
washing continued at pleasure; but the 
result of all the washings is sure to con- 
tain some gritty particles, wliich must be 
separated by repeated w^ashings till noth-' 
ing whatever will settle at the end of five 
minutes. Two sizes of crocus only are 
needed ; the last is obtained from tha 
I washed mass after one hour's suspension, 
' and is very small in quantity but of much 
