Monthlj' Microscopical"! 
Journal. April 1, 1869. J 
of Ohjed-glasses for the Microscope. 
227 
contact surfaces of one of the pair of lenses, the convex is deeper 
than the concave ; and bears hard in the middle. This may have 
no worse effect than loss of light ; but still it is as well avoided. 
On the Quality of the Glass employed in the Construction of 
Ohject-glasses. 
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 ingredients used, their propor- 
tions and chemical constitution, is in the hands of the makers ; and 
though the two or three of them that have paid attention to the 
manufacture have doubtless well studied the particular application 
of both the flint and the crown for the construction of microscope 
lenses, yet the best that we can procure falls far short of the 
requirements 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 liability to tarnish, and to all appearance 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 reflexion was much greater. 
Some ingredient had been added which increased the redaction, 
and probably lessened the dispersion ; and, of course, in a correct- 
ing concave, the latter quality alone is needed, and the lower the 
refraction the better. 
The crown and flint employed in the |th described at the com- 
mencement 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 proportion, and which when superposed were perfectly achro- 
matic. 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 the material elements of 
construction employed 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 equilateral 
prisms, and their refractive and dispersive powers tabulated, with 
the component ingredients. A few years back this investigation 
