THE YOUNG SCIENTIST. 
21 
Working diameter of bade crown, •325. 
Incident crown worlcing diameter, '34. 
These lenses must be worked up to a 
sharp edge at the above diameters, for the 
purpose of getting the requisite thickness 
of the lenses, which, in all optical com- 
binations of this description, is the only 
method by which it can be readily effected 
without the necessity of dismounting the 
lens from the lathe or stick on which it 
is ground, to gauge its thickness by means 
of a master-gauge. The finished diameter 
of these lenses is "Slo. Thickness of edge 
of flint at this diameter, '080. The middle 
combination of this objective is composed 
of two lenses— a double convex crown 
of '225, of equal curves, and a plano-con- 
cave flint. These, as a matter of course, 
like the back combination, are cemented 
together. The working diameter of the 
crown lens is '27, when worked to a sharp 
edge, as before described, the finished 
diameter being '26. Thickness of edge of 
flint, when reduced to the finished di- 
ameter, must be '070. The front glass is a 
plano-convex crown lens of '115 radius, 
which is worked to a half sphere ; its di- 
ameter, therefore, being equal to double : 
its radius. The combinations of this ob- ! 
jective, together with the front lens, are : 
set in their cells against shoulders, which ; 
renders the setting or centering of the! 
lenses, after a little practice, a very easy 
operation. The cell, carrying the front 
lens for convenience of adjustment, is 
best screwed to a piece of triblet-drawn 
tube, which can be made to fit over the 
oylindrical part to which the back and 
middle cells are screwed. By this con- 
trivance, the proper point of adjustment 
on mercury can be readily obtained, by ! 
shortening the plain end of the tube fitting 
until the diffraction rings become equal 
on both sides of the focus of the objective. 
The curves of this objective are computed 
to suit a density of flint 3'9646, crown 
12'540. This flint and crown glass is made 
by Messrs, Chance, of Birmingham, and 
-can be obtained from various dealers,* 
The posterior or back of the one-inch 
objective is comx:)osed of a triple com-j 
. I 
* The addresses of the principal dealers in 
optical goods and materials may be found in the 
* Microscopist's Annual." 
bination. The front or anterior is a plano- 
convex crown lens and a meniscus flint. 
The curve of the back surface of the pos- 
terior crown lens is '76. The curve of 
the next two surfaces is "525. Contact 
surface of incident crown, 3 "25. Inci- 
dent or outside surface, 2'5. The back 
surface of the posterior crown should be 
worked 8-lOths of an inch in diameter, to 
insure accuracy of figure. Previous to 
grinding the contact surface of this lens, 
the diameter must be reduced to '615, then 
worked to a sharp edge, which will give 
the thickness required for this lens. The 
working diameter of the incident plate is 
9-lOths. This, as matter of course, will 
be ground to a sharp edge, as before de- 
scribed, for the purpose of getting the re- 
quired thickness. The best method for 
an amateur to adopt would be to work 
the lens on a hard pitch pallet one inch 
or less in length. The lens is not so liable 
to tilt in working in the tool as it would 
be if the ordinary length of stick em- 
ployed for this purpose were used, This 
method also applies to the shallow sur- 
face of the fiint. Diameter of the pos- 
terior lenses when finished is 5-lOths; 
thickness of edge of flint at this diameter 
will be 1-lOth of an inch. Back surface of 
anterior flint is '41. Next two, or contact 
surfaces, '21. Working diameter of front 
crown, 4-lOths. Finished diameter "38. 
Thickness of anterior flint, at the flnished 
diameter, measuring from back surface 
to front edge, '125. Separation between 
the lenses is 7-lOths. Density of flint, 
3'64. Density of crown, 2'540. 
Curious Experiment Illustrating 
Inertia. 
THE experiment which we illustrate in 
the accompanying engraving, is a very 
remarkable one, and has the additional 
recommendation that it is very easily per- 
formed. Take a block of wood weighing 
ten to fifteen pounds, and suspend it by a 
}>iece of common cotton twine just as we 
have shown it. The block shown in the 
figure is a nicely turned ball, but this is 
not at all necessary ; any kind of a block 
will do. To fasten the twine, you may 
use a couple of screw eyes, or even good 
