THE YOUNG SCIENTIST. 
95 
circles in thousandths of an inch— for I 
thus note them down for the convenience 
of making and finding the steel gauges 
and to prevent divisions into two-thou- 
sandths, which would frequently occur in 
Fig. 12. 
the corrections. The following are the 
curves :— back triple,— posterior of crown, 
"312 ; three next surfaces, crown and con- 
cave flint, "440 ; front flat, diameter of lens, 
173 ; density of flint, 3 '630 ; ditto, of crown 
2-437. 
Curves or templates of middle: — Back, 
•233 ; contact surfaces, *233 ; front H inch, 
or Hhs inch radius ; diameter of lens, 138 ; 
density of flint, 3 "686; ditto of crown, 
2-437. 
Single front of crown, "100 or 1-lOth tem- 
plate; diameter of lens "093; thickness, 
'057, measured from the top; density, 
2-437. 
The focus, or magnifying power, of the 
two back combinations is very nearly 
equal, and each 41 times that of the single 
front ; for I have found that if the middle 
is of shorter focus than the back, that 
it is difficult to obtain satisfactory correc- 
tion. The lenses are fitted into their cells 
without shoulders, as their diameter is 
only just sufficient to admit the full pen- 
cil of rays, and their surfaces are utilised 
to the extreme edge, a desideratum that 
can always be secured by a proper mode 
of working. 
The aperture of this object-glass is 130°, 
which is amply sufficient for a good work- 
ing gth. In the triple back, the three 
cemented or contact surfaces are of the 
same radius, as I have not been able to 
ascertain that any material effect in the 
correction for spherical errors can be ob- 
tained by a difference in the two radii of 
the concave flint, and, therefore, for the 
sake of facilitating the workmanship, both 
faces are similar. I am aware, however, 
that some makers hold a different opinion 
and make the incident-surface of the con- 
concave much deeper, and the other 
longer in due proportion. The front of 
the triple is flat ; but as tl^e perfection of 
an object-glass depends in a remarkable 
degree upon the radius of this surface, a 
plano-convex lens cannot always be ap- 
plied as a rule, for the curvature depends 
very much upon the nature of the glass 
employed in the construction, and the 
distance at which the lenses are placed 
asunder. 
The correction for oblique pencils, and 
flatness of field, are mainly effected by an 
alteration in this radius, ascertained from 
the appearances of a globule of mercury, 
hereafter to be explained. Also, for the 
convenience of working, the posterior and 
contact surfaces of the middle lens are of 
smaller radii, and the required negative 
correction for color is obtained by an 
alteration in the concave incident-surface 
of the flint. The back and middle lenses 
are worked as thin as possible. It is an 
easy matter to make convex lenses to a 
sharp edge; but to insure the requisite 
thinness in the concaves, the edges are 
polished before the grinding is completed ; 
and this is continued till they are seen to 
be as thin in the centre as may be deemed 
practicable, without the risk of breaking 
them through. 
In the construction of the highest 
powers of the microscope, or such as are 
composed of three distinct sets of lenses, 
it must be borne in mind that the magni- 
fying effect is obtained principally by the 
front lens ; and the combined operation of 
the middle and posterior, is entirely cor- 
rective ; and their application in any com- 
bination must always be so considered, 
and not as a means of obtaining additional 
power. If the front of an eighth or one- 
twelfth is tested alone, it will be found to 
magnify nearly as much as when the 
other lenses are replaced. 
The single front has the advantages of 
facility of construction, and a command 
of any required extent of aperture ; and 
