Dr. Goring on Microscopes. 203 
because the principle itself is intrinsically bad, and incorrect at 
least for low powers. 
Now, if we form a microscopic object-glass of a single lens of 
considerable aperture, haying a stop in its focus of about the same 
diameter as the apertures of the common lenses used for com- 
pound microscopes, (that is to say, about one-tenth or one-twelfth 
part of their focal distance,) we shall form an object-glass which 
gives a clear image, free from fog indeed, but very deficient in 
other respects; for the stop being placed where the rays cross cach 
other, a large portion of the aperture of the lens is called into 
action, in comparison to what is usually made use of, when it is 
at once limited by a stop of the same diameter applied close to the 
glass; the aberrations both chromatic and spherical are here im= 
mediately felt—to remedy these, another lens must be employed, the 
best position for which is close to or very near the farther side of 
the stop. The focus of it must be to that of the first as 3 to 2, oras 
2 to 1—for low powers, however, it may be about 21 to 2—for the 
higher the best proportions seem to me to be as2 to1l*, The 
lenses employed should be plane convex, having their curves towards 
each other as represented in Figs. III, IV, V, and VI, Plate II. 
which are drawings of four object-glasses of this description which 
I have caused to be executed the lowest power is 2 inches focus, 
the highest 4 an inch—the foci of the lenses, and the size of the 
stops, Sc., are as there represented +. These object-glasses I can 
* The addition of this second lens has another good effect, for it enables 
us to regulate the compound focus so as to haye the object as near to the 
Object-glass as will consist with the distance which must be allowed for suf- 
fering the rays from a lens or mirror to fall upon it for the purpose of illumi- 
nation when opaque,—for the light of opaque bodies diminishes according to 
the square of their distance, and thus the farther the object-glass recedes from 
them, the less light it receives. With transparent subjects, however, the case 
is different, at least when they are illuminated by the converging rays of a lens 
orvoncave mirror ; for, by making the focus of this fall not upon the object but 
upon the object-glass, the maximum of light is obtained at whatever distance the 
object may be from the glass, so that the benefit of having them near each 
other is not so much felt as in the former case—the proportions I have recom- 
mended will answer every purpose. 
t It will be obvious that a microscope of my construction may be used as 
