18 Wenham_, 071 a Binocular and Single Microscope. 
for on bringing out the markings with the maximum dis- 
tinctness for the reflected vision, the direct will be found 
deficient. On altering the mirror, equal distinctness can be 
obtained in the direct tube, at the expense of the other. If 
each tube, therefore, has its own independent mirror, this 
inconvenience will be obviated. 
The adjustment for difi:erence of distance between the 
eyes is effected as before, the draw tubes being at the 
minimum limit of proximity when close in, and by drawing 
these out to a small extent they accommodate for all positions 
of eyesight. This answers so well in practice as to need no 
amendment. It will be seen that the reflected rays have 
further to travel to reach the eye-piece (the radius of each 
tube being the same). The distance is just equal to that which 
they have to traverse across the interior of the prism; this 
causes a slight disagreement of focus between the two, 
which may be compensated for by drawing out the main 
tube about a quarter of an inch more than the other, but it 
would be preferable to make a small diflerence in the mag- 
nifying power of the eye-pieces, which can be simply done by 
an alteration of distance between their lenses, each eye- 
piece to be marked for its appropriate tube. By transposing 
them such an adaptation would often compensate for those 
whose eyes differ materially in focus, or one being long and. 
the other short-sighted, which is a common defect. 
The base into which the prism slides rotates to a small 
extent, for reflecting the image level with the centres of the 
eye-tubes ; this is the only adjustment, and when set right is 
held fast by a binding screw in the side of the inner fine 
motion tube. The prisms having two opposite reflecting 
surfaces, possess the common property of such, that, how- 
ever much tilted, the direction of the ultimate emergent ray 
cannot be altered. Great care and nicety is, therefore, need- 
ful in working them to the exact angles for the definite 
direction in which the ray is to be finally reflected, but this 
having been properly obtained, gives the double-reflecting 
prism this advantage, that it cannot be readily put out of 
adjustment. Fig. 2 is an enlarged outline of the prism ; a 
ray of light, a, passing through the base, is totally reflected 
by the surface, b, towards c, at which surface it is again 
totally reflected in the direction required. Both the inci- 
dent and emergent surfaces of the prisms must be perpen- 
dicular to the direction of its corresponding ray, as any 
refraction is objectionable, and the reflecting sides be ar- 
ranged considerably within the angle of total reflection 
(which, for crown glass is about 48°) . The base of the prism 
