462 
POPULAR SCIENCE REVIEW. 
but plain directions how to use it; and it is hoped not only that 
a short paper on this subject may not be unacceptable to those 
who are thirsting to explore the boundless field of minute 
organic life, but that others may be induced by its perusal to 
possess themselves of an instrument with which they may 
thus at once, and with little difficulty, render themselves 
familiar; so that the band of scientific investigators may be 
reinforced, and the mass of valuable results be increased. 
The great optical principle which lies at the foundation of the 
Microscope is termed Refraction. By this term is expressed 
that change in its course which a ray of light makes in passing 
from one medium to another in an oblique direction. If a 
piece of stick be dipped perpendicularly in a basin of water no 
distortion appears, but the stick is seen to be straight as in the 
ah’. Depress, now, one end of the stick, and the part which is 
immersed in the water appears bent or distorted. The ray of 
light (or the stick) passing obliquely from the rarer medium 
(air) into the denser medium (water), is bent in such a manner 
as to approach towards a plumb-line let fall through the point 
where the air, water, and stick meet. It follows, therefore, 
that the ray which passes hi the other direction, namely, from 
the water (the denser medium) into the air (the rarer), is bent 
in the opposite direction, namely, away from the perpendicular 
plumb-line ; while the light which passes in the direction of the 
plumb-line itself, in or out, undergoes no contortion, bending, 
or refraction. The same thing that happens in the passage of 
a ray of hglit from air through water, takes place in its passage 
from air through glass, only glass being more dense than 
water, the refraction produced is greater. Taking advantage 
of this simple property of light, pieces of glass have been con- 
structed of such forms that the rays of light received by the 
Avhole surface of one side are so refracted that they all meet at 
one point (termed the focus) on the other side; the distance 
between the centre of such a piece of glass and the focus being 
termed the focal distance. Such a piece of glass, termed a lens, 
magnifies by enabling the eye to see the object at a shorter dis- 
tance than would otherwise be possible, thus causing the rays of 
light from the object to enter the eye at a wider angle, and 
thereby enlarging the imag’e impressed upon the retina. Those 
lenses which are convex on both sides, as an ordinary burning- 
glass, are most common; but in compound instruments, this form 
of lens is sometimes combined with others, as the plano-convex, 
plano-concave, concavo-convex, and double-concave. 
A common pocket lens consists of such a double-convex lens, 
which has the same enlarging effect as a glass-bubble filled 
with water, one of the most ancient forms of microscope. Two or 
three such lenses combined increase the effect. But the great 
