ILLUMINATION OF OBJECTS. 
about a foot in diameter, having a hole in its centre, through 
which the tube of the eye-piece is passed. This screen is then 
at right angles to the axis of the body of the instrument, the 
eye-piece projecting about an inch from it. The observer looking 
into the eye-glass with one eye, need not incur the exertion and 
fatigue of closing the other, since the screen performs the office of 
the eye-lid. 
The mirrors are sometimes made with a concave glass at one 
side, and a plane glass at the other, the latter being used when 
condensation is not required. A disc formed of plaster of Paris,, 
reduced to an extremely even and smooth surface, either plane 
or concave, is sometimes used with advantage when a soft and 
mild light is required. Nearly the same effect may be produced 
by placing a disc of white card upon the face of the mirror. The- 
illumination by a back light is attended with a peculiar advan¬ 
tage, inasmuch as it displays the internal structure of objects, 
and, in the case of organised bodies, supplies beautiful means of 
exhibiting the circulation; as, for example, the circulation of the 
blood in animals, and the sap in vegetables. In the case of certain 
animalcules, it shows some living and moving within the bodies of 
others. 
53. The following observations of Mr. Pritchard are worthy of 
attention:— u We must consider that in all bodies viewed by 
intercepted light, there is, properly speaking, neither light nor 
shade, in the ordinary acceptation of these terms; there are only 
dark and light parts, which again assume new aspects as the light 
is more or less direct or oblique. Thus depressions on transparent 
objects are almost sure, under the action of oblique light, to- 
assume the effect of prominences; but prominences seldom or 
never have the semblance of depression. As almost all diaphanous 
bodies can be examined as opaque objects, a scrutiny of them in 
this way will generally be found greatly to assist our judgment 
concerning their nature, whether they admit of being cut into 
sections or not. It would be easy to write a volume on this- 
subject only, if we commenced an illustration of particulars which 
could not be rendered clear and satisfactory without a vast number 
of figures. Long practice must, after all, determine our opinions, 
and scepticism should ever form a leading feature in them; we 
should suspect rather than believe . 
“ Opaque objects are not, upon the whole, so liable to produce 
optical deceptions as transparent ones, because we are more in the 
habit of viewing ordinary bodies by reflected or radiated light* 
The most common illusion presented by them is that of showing a 
basso-relievo as an alto-relievo ; the reverse deception sometimes 
occurs also, but more rarely. This effect occurs in ordinary objects- 
47 
