170 Construction of Object- Glasses [To^S^X 
III. — On the Construction of Ohject-glasses for the Microscope* 
By F. H. Wenham. 
(Continued from page 113, No. 11.) 
The aperture of this object-glass is 130° whicli is amply sufficient 
for a good working ^th. 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 obtained by a difference in the two radii of the con- 
cave flint, an,d 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- 
cave much deeper, and the other longer in due proportion. The 
front of the triple is flat ; but as the perfection of an object-glass 
depends in a remarkable degree upon the radius of this surface, a 
plano-convex lens cannot always be applied as a rule, for the curv- 
ature 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 similar radii, and the required 
negative correction for colour 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 ensure 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 magnifying effect is obtained principally by 
the front lens ; and the combined operation of the middle and posterior, 
is entirely corrective; and their application in any combination 
must always be so considered, and not as a means of obtaining 
* In the last number of this Journal the dimensions of the front of the ith 
appear as hundredths instead of thousandths. The cipher should be before instead 
of the end of the decimal, thus — " diameter of lens • 093, thickness • 057." I had 
overlooked this in the proof. — F. H. W. 
