94 THE > YOUNG 
Home-Made Telescopes and Micro- 
scopes— VII. 
GENERAL EEMARKS ON MAKING OBJECT- 
GLASSES. 
THE directions for working glass sur- 
faces to a correct figure, may appear 
to some too practical and characteristic of 
the workshop ; but it is only by a strict 
attention and study of such details that 
perfection can be insured, and without 
their aid, the deductions of the mathe- 
matician must fail in their proof. Though 
the early training of a mechanical profes- 
sion has familiarised me with such pur- 
suits, yet I must confess that I am igno- 
rant of the methods adopted by our best 
makers for working their minute object 
glasses ; and, therefore, if some particu- 
lars may have the merit of originality, 
others are perhaps not in accordance with 
the most improved practice. 
The first attempt to construct an object- 
glass (4 in.) is recorded in the year 1850, 
on the then well-known form shown by 
Fig. 11. The back lenses had an excess of 
Fig. 11. 
negative aberration, or were over-cor- 
rected, to enable the adjustment for cover- 
ing-glass to be performed by the separa- 
tion of the front lens, which was under- 
corrected for that purpose. But on at- 
tempting to improve the correction by a 
difference in the radius of the concave 
flint of the triple front, it was shown 
that a considerable alteration was here 
required to effect a material correction for 
color. Taking a ray at the focal distance 
from the front surface, and tracing its re- 
fraction through the triple, at all points, 
it appeared to enter the concave surface 
nearly as a radius from its centre. Con- 
SCIENTIST. 
sequently, under this condition, the effect 
of the dense flint was partly neutralised. 
It then occurred to me to try a single 
lens for a front. With this combination 
no satisfactory result could be obtained 
with respect to achromatism. 
Early in the year 1850, Mr. Lister was 
occupied with experiments for the purpose 
of improving the higher powers, and then 
introduced the triple back, which has 
since so eminently proved to be the 
grandest step towards their perfection, 
allowing perfect correction to be obtained 
with the most extreme apertures. 
Having received early information of 
this improvement, I set to work and again 
tried the single front in combination with 
the triple back, and constructed a ith on 
this system, which is considered excellent 
to this day. For several years I stood 
alone in my opinion of its advantages ; but 
as numbers of our best object-glasses of 
the highest powers are now made with 
single fronts, I am in a better position for 
advocating this form, particularly as its 
success was found to depend upon a rela- 
tive difference of focal lengths in the two 
back combinations not hitherto employed 
by others. 
At first the single front with the back 
triple was not successful. Though color 
was nearly corrected there was a defici- 
ency of aperture, and the combination 
was spherically under-corrected. On 
viewing another object under a thicker 
covering-glass, the definition was greatly 
improved. By placing other pieces of 
thin glass over the object, the front lens 
had to be drawn still closer to the others. 
This gave an increase of aperture and 
more perfect definition. A single front 
was then made, of the thickness which 
had been found to give the best result, 
ascertained from the measurements of the 
additional pieces of thin glass over the 
object, and the effect was all that could 
be desired. On finding that the correc- 
tion for spherical aberration depended 
upon the thickness of the front lens, the 
path became easy. 
Fig. 12 represents a §th of 130° of aperture 
constructed on this system, six times the 
size of the original. The curves are not 
given as radii, but the diameters of the 
