106 Wen HAM, on a Binocular Microscope. 
needed in adjusting the small prism so as to get botli reflec- 
tions combined, otherwise a blurred image will be seen in the 
slanting body. 
As to the performance of this arrangement the introduc- 
tion of the prisms do not perceptibly interfere with the 
definition in the direct tube — delicate markings may be seen 
quite as well with the prism in place as out, and with proper 
workmanship the result is equally good in the other j and 
having the privilege of using both eyes, the observer is 
enabled to view diatoms and other minute objects more 
agreeably, and with a better idea of their structure^ than with 
single vision. The great defect is a want of stereoscopic 
effect arising from the near similarity of the images. In 
this respect it falls far short of the common prism in the 
qualification of giving a perspective view with the lower 
powers. 
In most arrangements constructed on the principle here 
described it is immaterial at what distance behind the object- 
glass the prism and reflecting surfaces are situated; the 
definition will be equally good, and the field entire, if fixed 
close to the eye-pieces. 
The rectangular prisms arranged with their diagonals in 
contact may be otherwise made serviceable for the micro- 
scope. With some plan of modifying the light it may be used 
for a camera lucida and also for an illuminator in place of 
the glass disc employed where the object-glass acts as its own 
condenser for opaque objects. ^ 
On this principle of illumination the right system appears 
to be to obtain a very intense parallel ray of light, of as small 
a diameter as possible, thrown through a portion only of the 
object-glass. 
With these prisms it is possible to obtain totally reflected 
light for the illumination, at the same time that the image of 
the object is transmitted. The light for the illumination 
must in this case be sent through the object-glass somewhat 
obliquely, and to reduce the diameter of the ray down to the 
small size required a stop of tinfoil may be atached to the 
lateral surface of the prism. 
