Wenham on Binocular Vision. 
7 
Fig. 6. 
see the day when mj own designs for effecting the same end 
may be rendered obsolete by the march of improvement. 
In the plan just referred to, the error arising from the 
length or thickness of the prisms may 
be diminished by making the two mi- 
croscope bodies converge towards b b, 
fig. 3, and adopting two rectangular 
prisms with the reflecting surfaces at 
the proper inclination for directing the 
rays of light from the object-glass, up 
the centre of each tube : by this means 
we can much reduce the substance of 
glass that the rays will have to pass 
through. 
Figs. 5 and 6 represent another 
method that I have contrived for using 
only two prisms, which can be made of 
the smallest possible size, and also at 
the same time do away with one re- 
flecting surface, which is one of the 
principal sources of error. Fig. 5 is 
the plan, and fig. 6 the elevation, a, 
fig. 6, is the object-glass, over which 
are two right-angled prisms 5 Z>, placed 
side by side with the reflecting sur- 
faces, at an angle of 45°. c c, fig. 5, 
are two Huygenian eye-pieces, placed 
a sufficient distance asunder to suit 
the eyes, and converging towards the 
vertical axis of the object-glass. The 
contact sides of the prisms must be 
equally ground away, till their two 
emergent surfaces are in a plane at 
right angles to the axes of their respec- 
tive eye-pieces. This method in- 
volves the necessity of having the 
object-glass at right angles to the 
two bodies of the microscope, and is therefore just suited to 
some of the foreign form of stands, but is of course inap- 
plicable to the English ones, unless, indeed, we mount the 
bodies after our usual fashion, and allow the object-glass to 
point upwards in an inclined direction, and pass through the 
bottom of a stage, on the upper surface of which the objects 
can be placed, and which surface should be parallel to the 
axes of the bodies. This would give great facility for direct 
illumination ; for whether we used a superposed achromatic 
condenser or not, we should not require a mirror either by 
