THE ELEMENTS OF MICROSCOPY. 51 
The following diagrams will perhaps explain this,— 
When working with non-achromatic plano-convex lenses,—such 
as the bull’s-eye condenser, for instance,—it should not be over- 
looked that the spherical aberration is always less when divergent 
or parallel rays fall upon the curved side of the lens. 
We may now be able to comprehend why the term sidereal 
focus, or, simply, focus, has been adhered to so long: parallel rays 
falling upon lenses of varying curvatures yield foci at definite dis- 
tances from the lenses, and we shall now see that the magnifying 
power of a lens depends upon the curvatures of its surfaces. A 
plano-convex lens, ground to a curve of half-an-inch radius, would 
give us a one-inch lens, magnifying ten diameters, while a double 
convex lens, ground to the same curves, would be a half-inch lens, 
magnifying twenty diameters ; not, be it understood, that these 
lenses would focus at one inch, or half-an-inch respectively from 
the object, but, that when solar rays are transmitted through them, 
the focal point is situated at those distances from the outside sur- 
face, as shown upon the screen. 
Last to be considered is working distance, and from the diagram 
you will be able to see that the thickness of the lens must affect 
this very materially. If the thickness of the lens before you be a 
quarter of an inch, the working distance will be a trifle over three 
quarters of an inch. The thickness of a lens does not alter its 
magnifying power. 
Having, then, for our guide the fact that the working distance 
of a plano-convex lens, with its plane side to the object, is that of 
its sidereal focus, less the thickness of the lens, and plus a small 
quantity due to the reduction of the posterior focus from infinity 
to about ten inches, we shall see that if the lens is a hemisphere 
the working distance will be approximately half its focal length. 
This is true for single lenses only, as the back combinations of an 
objective require the approach of the front lens to the object much 
nearer than the above rule would indicate. 
My paper having now exceeded the bounds at first planned for 
it, I will conclude for the present, and if you desire to hear more 
I shall feel happy in giving you another paper ‘‘ On the Construc- 
tion and Use of Objectives,’ in which I will deal with the 
aperture question. 
