( 'i'4 ) 
Art. XII . — Account of some Single Microscopes upon a new 
construction. By David Brewster, LL. D. F. R. S. Lend, 
and Sec. R. S. Edin. 
There are few instruments which have remained so long 
unimproved as the common single microscope. During the last 
200 years, it has undergone almost no change, and it can 
scarcely be doubted that the magnifying lenses employed by 
Leeuenhoek were equal to any that are used in the present day, 
although our artists may fit them up with more elegance ; — and 
by nicer apparatus, and superior modes of adjustment, may ac- 
commodate them better to the purposes of scientific discovery. 
Any contribution, therefore, to the improvement of the single 
microscope, however small be its amount, will not be altogether 
unprofitable, if it has no other effect than to shew that the in- 
strument is still susceptible of interesting modifications, and that 
the attention of ingenious individuals may be directed to its im- 
provement with some hopes of success. 
In the annexed diagram, I have represented a method of 
using a plano-convex lens., so as to obtain from it twice the mag- 
nifying power which it possesses when used in the common way. 
If ABC be a hemispheri- 
cal plano-convex lens of half 
an inch radius, it will magni- 
fy about fourteen times as a 
single microscope; the dis- 
tance at which the eye sees 
minute objects most distinct- yv 
ly, being taken at 7 inches. 
Let us now suppose that the 
lens is placed, as in the figure, with a microscopic object at R, and 
the eye at e, the rays issuing from the object R, will, after refrac- 
tion at the surface BC, fall upon the plain side AC, from which 
they will be reflected at the points «, 5, c ; and after a second re- 
fraction at the surface AB, they will emerge parallel to one ano- 
ther at d, e andy, provided the object R be placed as far before 
BC as the anterior focus of a double convex lens of the same 
curvature as BC. 
