OPTICAL IMAGES. 
20. Thus, for example, if the transparent medium be water, the 
depth of the image will be about three-fourths of the depth of the 
object, and consequently water, when the bottom can be seen, 
always appears less deep than it is in the proportion of 3 to 4. A 
reservoir, whose real depth is 12 feet, will appear to have a depth 
of only 9 feet. 
If the transparent body be glass, which has a greater refracting 
power than water, in the proportion of about 8 to 9, an object 
attached to the under-surface will appear to be at the depth of 
about two-thirds of the thickness of the glass. 
21. If a rod L b l', fig. 12, be plunged obliquely in water, it 
will appear as if it were broken at b, the part immersed being 
seen, not as it really is in the 
direction b l, but in the direc¬ 
tion b l\ This will be easily 
understood, when it is con¬ 
sidered that the image of such 
point of the rod will appear at 
a less depth than the point 
itself, in the proportion of 3 
to 4. Thus the image of the 
several points p will be at 
the points p , the depths 
being severally three-fourths of the depths m p. 
22. A certain part of the light which strikes upon the surface 
of a transparent body will enter it, no matter what be the obliquity 
with which it encounters it; but there is a certain obliquity beyond 
which light cannot emerge from it. Thus a ray of light proceed¬ 
ing from any object under water, which strikes the surface at an 
angle less than 41° 32', cannot emerge, and in that case it may be 
asked, what becomes of the ray ? The answer is, that it will be 
reflected back into the water exactly as if the surface were a per¬ 
fectly polished plane surface. 
In the same manner, if the transparent body be glass, the ray 
cannot emerge from it, if the obliquity be less than 48° 11', and in 
this case the ray will be reflected. 
The reflection which takes place under such circumstances, is 
much more complete than any reflection from the surfaces of 
bodies, whether naturally smooth or artificially polished. It has, 
consequently, though somewhat improperly, been called perfect 
reflection, for, although the reflection is incomparably more 
perfect than that from smooth or polished surfaces, nevertheless 
there is still a small part of the light lost. 
The angle which limits the obliquity at which light can emerge 
from a transparent body, is called the limit of transmission. 
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