106 
THE YOUNG SCIENTIST. 
the tools in such a manner that no serious 
accident will happen. Mrs. Carpenter also 
suggested that, as the boys were going to 
be taught by their father to make useful 
things, it would not be out of place if she 
undertook to give Jessie a few lessons now 
and again, in making such things as will 
be necessary to dress her doll in good 
shape, and decorate and properly finish 
the interior of her play-house, which Fred 
will no doubt build for her. Mr. Carpen- 
ter thought the idea a good one, and it 
was decided at once that the boys should 
have a workshop and tools, and should be 
taught how to use the latter, and that 
Jessie should learn housekeeping on a 
small scale — in a play-house — and many 
other things that a good little girl ought 
to know. 
Reader, you and I will watch these three 
young people while at work. We may be 
able to learn something from them or 
their instructors. 
Let us see ! 
(To he continued.) 
Home-iTiade Telescopes and Micro- 
scopes.— IX. 
GENERAL REMARKS ON MAKING OBJECT- 
GLASSES. 
IN May, 1856, I made the first l-25th 
with a single front lens of l-30th in 
diameter ; I am doubtful whether a triple 
front could be made even of this size, 
with any positive certainty of accurate 
•workmanship. 
From the g and upwards, perfect correc- 
tion may be secured with a single front. 
It is, however, barely possible to make a 
good l-5th with this form, and in a 2-inch 
it fails altogether ; there is a kind of sec- 
ondary spectrum that cannot be got rid of. 
It is not easy to define all the reasons 
why it should succeed perfectly with the 
highest powers, and the correction be im- 
perfect with the lower ones named. With 
smaller apertures the errors of spherical 
aberration cannot be so well corrected by 
giving thickness to the front lens ; and as 
there is considerable distance between 
this and the middle, the colored rays from 
the uncorrected front are so far separated 
that any corrective action of the back sys- 1 
terns is incapable of recombining them. 
When an object-glass is spherically under- 
corrected, the focus of the central rays is 
longer than that of the marginal ones, as 
in a simple lens. If all the rays are 
brought to one point, the interposition of 
a plate of parallel glass projects the out- 
side rays to a greater distance than the 
central ones, and produces a similar effect 
to a concave lens, or that of over-correc- 
tion ; and it is for this reason that a 
certain thickness of glass before, or in the 
substance of the front lens has such a re- 
markable corrective power, which is most 
appreciable with a very large aperture. 
Where this is comparatively small, as in a 
5-inch, the influence of a thick front does 
not appear to be sufficient to enable the 
final correction to be obtained by this 
means alone. The anterior lens must,, 
therefore, either be partly achromatised, 
or made of a glass of higher refractive and 
less dispersive power than any at present 
known. 
It is well known that, in a doublet con- 
sisting of two single plano-convex lenses, 
both the spherical and chromatic aberra- 
tions are considerably less than in a single 
lens of the same magnifying power. I 
have for this reason proposed to construct 
the higher powers with two single lenses 
