Wenham, on a Binocular and Single Microscope. 17 
Pig. 1. 
main tube. The additional body may either be soldered 
permanently on to the other, or be made to draw on and off, 
a double collar holding them 
together at top, and a clip or bolt 
at the bottom. In the latter case, 
when the inclined tube is re- 
moved, a cover should drop 
neatly into the aperture, flush, 
and be secured by a bolt. But 
the additional body being no 
hindrance to the ordinary action 
of the microscope, it is best always 
to allow it to remain in place ready 
for instant use, as required. When 
the prism is drawn out to its limit, 
the main body acts just as the 
usual single instrument, and 
therefore needs no explanation ; 
but on thrusting it in, a part of 
the rays are thrown into the eye- 
piece of the inclined body, and 
thus the right-hand rays of the 
object-glass are reflected into the 
left eye, and the remainder pass 
directly into the right eye, having 
nothing intervening to obstruct 
them in the due performance of 
their best effect. The prism 
need not, in all instances, be 
thrust in to its fullest extent, so 
as to take in the total half of the 
object-glass, but only partially, 
to the degree requisite for throw- 
ing the object up in relief. In the case of a difficult test 
the largest share of the direct aperture may be employed, 
while, by coaxing the illumination for the reflected portion, 
the instrument can be made to perform well on the diato- 
maceous tests. 
With respect to the illumination, in all cases where pos- 
sible the opaque principle should be employed, as it gives 
to objects a far more natural appearance. When transmitted 
light is needed, a large, angular pencil should be used, other- 
wise the two fields cannot be equally lighted with the higher 
powers. I intend to have a split mirror made, each half 
capable of separable and independent adjustment for each 
body. The necessity for this will be shown with the Podura^ 
VOL. I, NEW SER. h 
