102 NATURAL HISTORY MISCELLANY. 
the light except these excesses should be extinguished, it will appear at 
once that the illumination still outstanding would be such as is required 
to produce stereoscopie vision; that is, each half of the pencil would go to 
the opposite eye. In our calcite prism, we have seen that in, for instance, 
nary ray, on the other hand, is almost totally transmitted between a! and 
c', and loses something by reflection toward b’. These effects are more 
marked in some of the oblique pencils, and the consequence is, that, with 
low powers, the stereoscopic appearance is very perceptible. O cross 
the reflected rays upon the transmitted behind the prisms would there- 
fore be productive of a pseudoscopic effect which would be objectionable. 
But with high powers, on account of the small difference of incidence 
existing in that case between bb! and cc!, the image appears plain. 
and with the B oculars, the Providence Grammatophora is thus resolved 
with great facility. 
When the power used is below one-fourth, there is a little haziness pro- 
duced in the image seen by reflection, in consequence of the mingling of 
the, to some extent, reflected extraordinary ray, from the clear field sur- 
rounding the object. This effect is immediately removed, by placing over 
the slide a card, out of which has been cut a slip having the width of the 
field. Such a card, or a similar thin plate of metal, may be easily secured 
to the stand, so that the stage and slide may move beneath it while it 
mains fixed. This haze is moreover suppressed still more easily by 
_ Slightly tilting the system of prisms, so as to diminish by a degree or two 
the angle of incidence upon the reflecting plane of section. ‘This really 
gives to the image seen by transmission the advantage in respect to il- 
lumination; but as, with low powers, both images are strongly illumi- 
mounting the prisms, to provide some system of adjustment by whi e 
position may be varied to correspond to the power emp 
e ex me ave been made with calcite prisms cut in such a 
manner that the extraordinary ray p ding from common light pe t 
reflection is ábout the same as before. The construction employed at 
first gives results which are very satisfactory; but it is designed to pur- 
