698 NATURAL HISTORY MISCELLANY. 
by a large increase in the amount of light lost at the second reflecting 
surface, and by a corresponding diminution of the brightness of the im- 
-— seen pe transmission 
oculars estre d on.the principles of those last described may be 
SHE cata-dioptric, in contradistinction from those which split the body of 
the light geometrically, and which are properly denominated stereotomic. 
ey have not the advantage which belongs to stereotomic binocular 
of presenting the object viewed in all its three dimensions. But re 
pennt what most observers regard as very desirable, or find at least very 
comfo dp the use of both eyes at the same time. It is true that there 
are many whom practice has so accustomed to the use of a single eye, 
that vam profess to suffer no inconvenience from this mode of observa- 
tion, and regard binoculars with indifference except so far as they are 
BARE A by their stereoscopic effect. But ; however slight may be 
for a series of years, without finding that his eyes have lost the equal 
power which they once possessed of accommodating themselves to dis- 
tances. It seems iy iy sible to prevent this result from supervening 
sooner or later, unless by maintaining a strict impartiality in the employ- 
ment of the eyes praia at the microscope; and this is what few re- 
a good form of this instrument adapted to the higher powers desirable. 
Such a form is believed to have been found in the construction now to be 
described. 
If a rectangular prism of calc spar be cut with four of its faces parallel 
and the other two perpendicular to the direction of the optic axis, a ray 
of light incident perpendicularly upon any one of the lateral faces will be 
divided by double refraction into two rays, but both of these two rays will 
plane. And the diner ray will 2 mex. total reflection at an angle at 
transmission occurs for the extraordinary ray is e P From M^ 
th 
ray originally incident upon the prism. If, therefore, the supposed calc 
spar prism were cut by a plane, making an angle of 37° 11' with one of its 
lateral faces, a ray incident perpendicularly upon this lateral face and 
meeting the plane of section, would be half reflected and half transmitted, 
