Dr. Goring on Miéroscopes. 207 
Thave several much deeper made on this plan up to ;'5 inch—all 
of which shew the parallel lines in question, and other equally 
difficult objects,;—the deepest lenses have their apertures some~ 
what larger than this ratio, for the sake of the light, (for it appears 
that you may increase this aperture, though you must not diminish 
it, and yet see these objects, though the fog then becomes very 
great and disagreeable; the colour also grows very apparent on 
account of the largeness of the aperture relative to its focus. 
Common microscopic object-glasses as we all know are sufficiently 
achromatic with the small apertures, and the shallow eye-glass 
of one inch focus usually employed, in which respect there is an 
analogy between them and telescopes with object-glasses com- 
posed of single lenses of small aperture, and a shallow eye-glass. 
The achromatics only differ from them in carrying a larger aper= 
ture witli a deeper eye-glass, which again have their limits, beyond 
which the colour appears as before, 
_ In Figs. VII, VIII, and 1X, are representations of some silver 
cups for holding very deep single lenses intended to view opaque 
objects, which, together with the object-glasses before-mentioned, 
were executed for me by Mr. Tuther, optician, in High Holborn, 
to whose politeness and skill I am indebted for being able to carry 
my intentions into effect. It is generally supposed that single 
lenses will shew objects perfectly clear and without fog, but this 
is not the case unless their apertures are very small,—lenses of 
as ao 5, and ;!,th of an inch focus require their apertures to be 
so much reduced to shew opaque objects clearly, that it is scarcely 
possible to see at all with them from the want of light. These 
cups were contrived to remedy this defect as far as it is practi- 
scope would be, having an object-glass of the same aperture with the microscopic 
one with a focal length, equivalent to the distance between the object-gluss of the 
microscope, and the focus of its eye-glass having its image magnified by the 
said eye-glass. For example, I measured the power of a microscope in the 
legitimate way with two similar micrometers, one on the stage, the other at 
the field bar in the focus of the eye-glass—supposing the eye-glass of 1 inch 
focus to have magnified six times, the power was 386, while the size of the 
pencil at the eye-glass compared with the diameter of the object-glass was 
merely as 2 to 6—the oue being 375 of an inch, the other y¢5- 
Q2 
