T6 Dr Brewster on some Single Microscopes 
this construction is very remarkable, as we obtain a better effect, 
from a single hemispherical lens with one convex and one plane 
surface^ than we do from a prism and two plano-convex lenses^ 
having five plane surfaces and two convex ones. 
The ingenious idea of Dr Wollaston to render single micro- 
scopes periscopic, or to extend their limits of distinct vision, by 
placing a circular aperture between two plano-convex lenses, as 
shewn at w /i in Fig. 1. of the annexed diagram, has suggested^ 
several improvements on the single microscope. 
Fig. 1. Fig. Fig. S. Fig. 4* Fig. 5. 
m m m m m 
is lost by doubling the number of surfaces ; but that this is 
more than compensated by the greater aperture, which, under 
these circumstances, is compatible with distinct vision In 
order to remedy this evil, we propose to combine the lenses, as 
in Fig. where the aperture contains a fluid of nearly the 
same refractive power as the glass, namely, oil of turpentine, 
castor oil, or Canada balsam, which will also act as a cement, in 
holding the lenses in their place. 
The same effect may be produced more perfectly in a sphere, 
or in a very deep double convex lens, by grinding a groove in 
its circumference, as in Fig. 3. which could be easily done by 
the wheel of a seal-engraver. In this way the doubling of the 
surface is avoided, and those sources of error removed which 
arise from the imperfect centering of four surfaces, and the 
combination of the two lenses. 
If we wish to diminish the aberration of colour, the two 
lenses used in Fig. 1. and % may be double convex ones, and 
having their anterior surfaces of such radii, as to form a con- 
cave lens in the aperture when it is filled with a fluid of a diffe- 
rent refractive and dispersive power from the glass. This com- 
Fkil Trans. 1812, p. 376. 
