208 Dr. Goring on Microscopes. 
cable ;—their radius is only + of [an inch, their focus consequently 
4, These condense light much more than the larger cups.com- 
monly used, and illuminate much more powerfully. It is true 
that they only enlighten a small portion of an object, but then we 
can only see a very small portion with such deep lenses as they 
are intended to hold; they are not so small but that they may be 
made to receive and condense the whole of the light proceeding 
from a bull’s-eye lens placed at a proper distance from them, and 
in this way with no other light than that of a common candle, I 
have been enabled to see well an opaque object with a compound 
microscope, having an object-glass of only Aj inch focus set in 
one of them, with only a moderate aperture. A lens of ;4, inch 
set in this manner, used as a single lens, likewise shews opaque 
objects in a manner which leaves nothing to be desired. 
I must mention, however, that it is necessary for the stops 
between which the lenses are placed to be very accurately made. 
They should be turned out of a piece of solid brass, the external 
one very thin, and the holes so correct as always to coincide with 
each other when the stops are turned round; the apertures must 
be quite free from burrs ; in addition to which the stops must be 
so adjusted that the focus of the lens and that of the cup must 
precisely correspond, otherwise the benefit of the cup is in a great 
measure lost.- Fig. IX will carry 4 or 45 of an inch without any 
stop at all, which is a great convenience, for the lens is in this 
case close to the eye, and the field of view larger in consequence : 
the stops for the deeper lenses are much shorter than they would 
be with larger cups, (Figs. VII and VIII,) so the field is increased 
in the same way, and the eye much less strained in using them 
than it would be were the lens farther off from it. I have shewn 
many individuals objects with the 4, inch lens not remarkable for 
the strength of their eyes, who saw with perfect ease, and were not at 
all conscious of the extreme smallness and depth of the lens they 
were using. As single lenses are generally considered to be most 
adapted for making discoveries in natural history, as being less 
likely to create optical deceptions than compound magnifiers, I 
imagine I am doing naturalists a service in putting them into a 
