204 Dr. Goring on Microscopes. 
confidently recommend as greatly superior to those in common 
use; they are bright, clear, and distinct, free from spherical aberra- 
tion, and will shew no sensible colour with opaque objects of any 
kind, not even with so trying a one as the enamelled white 
letters on a black ground generally used by opticians to try their 
telescopes with. When, however, they are made to view an object 
illuminated from behind, which does not suffer the light to pass 
through it while its edges are seen, as for example the legs of some 
insects, some kinds of moss, &c., which have very little trans- 
parency, the uncorrected colour is then decidedly seen—such 
objects are the best tests of achromaticity for telescopes as wel) 
as microscopes ; equivalent terrestrial ones for a telescope will be 
the bars of a window seen from the interior of the apartment to 
which it belongs, or the naked branches of a tree in winter, seen 
against the light of the sky, more especially of the sun, and 
nearly opposite the observer. In addition to the four object- 
a magnifier for a telescope, In factit is in its principle nothing but the four 
glass erecting achromatic eye-piece of a day telescope a little modified (there 
is alas nothing new under the sun). Indeed, many of Mr. Tulley’s astrono- 
mical telescopes are so constructed that the night eye-pieces can be applied 
to magnify the erected image formed by the two glasses, which do the work of 
my object-glass. It would, however, be much better, instead of increasing 
the depth of the eye-glasses in this case, to augment that of the erecting part, 
as a much sharper image is in this way obtained. There certainly are many 
objects which are seen better with this kind of eye-glass, such as Venus, and 
many double stars ;—the number of refractions arrest a portion of the false 
light or halo which so commonly surrounds these objects. However, the 
same or nearly the same effects seem to be produced, by diminishing the 
aperture of the object-glass of the telescope, except that this seems to increase 
the spurious disc of the fixed stars, which the other method does not. Many 
suppose that great advantages are to be gained by making a microscope 
with a long tube, and a shallow eye-glass. I have satisfied myself repeatedly 
by experiment, that whether the required magnifying power is obtained in 
this way, or by a short tube with a deep eye-glass, the effect is precisely the 
same. The body of my microscope is seven inches long, having an achro- 
matic eye-piece of about one inch negative focus, just like those applied to 
telescopes. I do not like the double and triple eye-glasses very commonly 
applied to microscopes, as they are apt to give double images, with luminous 
transparent objects, 
