THE YOUNG SCIENTIST. 
3 
optics. Having gone thus far the reader 
will be able to guide himself, and the 
further he goes in this direction the better 
work will he be able to turn out. 
To be able to make the lenses for a 
microscope or telescope we must have: 
First, a knowledge of the proper forms of 
the pieces of glass that are needed, and 
also of the qualities of the kinds of glass 
best suited to the different parts; and, 
secondly, the mechanical ability to work 
these different pieces of glass into i^roper 
shape. The second requisite is that which 
in practice makes itself felt first, and we 
shall, therefore, turn our attention at once 
to the materials and tools required. 
GLASS. 
Glass is the material out of which all 
lenses for purely optical purposes are now 
made. Chemists enumerate nine different 
kinds of glass, but of these two only, fiint 
glass and crown glass, are generally em- 
ployed by opticians. Speaking of the glass 
used in making the best modern objec- 
tives, Mr. Wenham gives us the following 
information, much of which will at present 
be beyond the understanding of the ma- 
jority of our readers, but, as we desire to 
make these articles complete and furnish 
information which will be valuable to ad- 
vanced students, we give it unabridged. 
He says : 
"Under this head I can offer but very 
little information, for, in common with 
all other workers in this direction, I 
have merely made use of such various 
samples of glass as I have been able to 
procure. The whole secret of the in- 
gredients used, theii; proportions and 
chemical constitution, is in the hands of 
the makers ; and though the two or three 
of them wdio have paid attention to the 
manufacture have doubtless well studied 
the particular application of both the flin t 
and the crown for the construction of 
microscopic lenses, yet the best that we 
can procure falls far short of the require- 
ments of the case for the very highest 
powers. 
''It is usual to denote the quality of 
flint glass by its density, but this in 
reality forms no accurate criterion of its 
dispersive power. Formerly, under this 
impression, I procured a quantity of dense 
flint, made by Chance, of Birmingham — 
very hard, white, and free from ability to 
tarnish, and to all appearances as good a 
quality of glass as I had seen. Its density 
was 3 '867, but on trial I found it unfit for 
the construction of the highest powers, as 
its dispersive power was lower than the 
Swiss 3-686, or even the 3-630 that I had 
employed previously while its refraction 
was much greater. Some ingredient had 
been added which increased the refrac- 
tion, and probably lessened the disper- 
sion ; and, of course, in a correcting con- 
cave, the latter quality alone is needed, 
and the lower the refraction the better. 
"The crown and flint employed in the 
one-eighth described at the commience- 
ment of this essay,* of the respective 
densities of 2-437 and 3 '686, had a relative 
dispersive power of 11 to 25 ; this having' 
been very accurately determined by two 
prisms, whose angles were in this proi:)or- 
tion, and which, when superposed, were 
perfectly achromatic. Faraday made 
some dense flint having a specific gravity 
as high as 6 '4, but we have no information 
relating to its refractive and dispersive 
power. 
"We are thus somewhat ignorant of 
I the material elements of construction em- 
! ployed in the microscope object-glass ; and 
it would be very desirable that a series of 
experiments should be made, with various 
combinations of all the known materials 
that can be used in glass-making, and the 
resulting compounds worked into equi- 
lateral prisms, and their refractive and 
dispersive powers tabulated, with the 
component ingredients. A few years back 
this investigation w^ould have been a very 
troublesome and expensive one, by reason 
of the interference of the Excise laws, and 
the necessity of employing a regular glass 
furnace, to operate on large quantities at 
once, in order to lessen the effects of im- 
purities. But now, by means of the re- 
cently invented gas furnaces, the greatest 
possible heat may be commanded, under 
perfect control, and thus enable the oper- 
ator to combine materials in very small 
*In a subsequent number we shall publish Mr. 
WeJiham's directions, illustrated with cuts, for 
making this objective. 
