WenhaMj on an Improved Binocular Microscope. 155 
Fig. 2. 
prisms, on its final emergence from the crown, is refracted 
inwards instead of outwards, as 
in the former prism (of which 
this is the converse), and the 
right and left-hand images from 
the object-glass cross each other 
to the opposite eye, invariably 
giving a true orthoscopic effect. 
The dispersive power of the 
prisms must be balanced, to 
obtain freedom from colour; 
and the degree of refraction 
such as to throw the image of 
a particle, ^.^fltc//// into the centre 
of each eye-piece. 
The second defect has also been overcome in the following 
manner. The two bodies are still permanently held at one 
fixed angle of convergence towards the object-glass ; but the 
angle has been diminished to somewhat less than that de- 
CROWN 
h 
c c 
Fig. 3. 
scribed in my first instrument, 
so as to bring the eye-pieces 
nearer together to meet the re- 
quirements of those whose eyes 
are set in more than usual prox- 
imity. The two bodies are 
furnished with draw-tubes, by 
lengthening which the distance 
between the two eye-pieces is 
increased to suit the eyesight 
of persons whose vision is far 
apart. By drawing out the tubes 
two inches, I can vary the trans- 
verse distance from 2e to 2| 
inches, which range is amply 
sufficient for all variations of 
ocular position ; and the tubes 
being elongated in the line of 
each axis, an object always 
maintains its position in the 
centre of the field. Fig. 3 repre- 
sents the outline of the bino- 
cular microscope: a a, object- 
glass; h, prism; cc, rays from 
object-glass crossing over to 
opposite sides on leaving the prism ; dd, eye-pieces. 
In conclusion, I may state that the thinness of the achro- 
matic refracting prism gives it a great advantage in the 
