126 
NATURE 
Dec. 19, 1872] 
The discoveries of Newton, to which I have already 
alluded, were connected with prisms, and were based on 
well-known properties of light, 
If a beam of light, as for instance sun-light or an 
artificial white light, be allowed to enter a dark room 
from a round hole in a shutter, it will simply travel in a 
straight line from its source ; and to make it deviate from 
this straight line one of two things must be done. The 
beam must either be reflected or refracted. ; 
The reflection of light is of very ordinary occurrence, 
for when light strikes any polished metallic surface, or in 
fact a surface of any kind, it is more or less reflected by 
it. The phenomena of reflection are so well known, the 
perhaps one of 
use of the mirror or looking-glass being 
Fic. 5.—Refraction of light. Apparent elevation of the bottoms of vessels. 
the most tangible, that no detailed reference need be 
made to them. The refraction or bending of light takes 
place when the ray passes obliquely from one medium to 
another of different density, as from air into water, or from 
water into air. A simple experiment may be made by passing 
the beam of light from above into a glass vessel containing 
water. Ifthe ray strikes the surface perpendicularly, it will 
be seen that no visible change takes place, the ray simply 
proceeds directly into the water without altering its direc- 
tion. If, however, the beam be allowed to fall on the 
surface of the water, say at an angle of about 45°, two 
things may be observed. In the first place a reflection 
will take place at the surface of the water—that is to say, 
the light will appear reflected at the surface, and it will 
be noticed incidentally that the angle at which the re- 
flected ray leaves the water is precisely equal to that at 
which the incident ray strikes the surface, thus proving 
the rule that “the angle of incidence and of reflection 
Fic. 6.—Light passing through plate of glass. 
are equal.” The second thing to be noticed is that on 
entering the water the direction of the beam of light will 
not be the same as it was inthe air. In Fig. 3, the ray 
RI, striking the water at I, instead of proceeding to R,, 
is deflected or refracted to S ; that is, the ray will be bent 
downwards, or, what is the same thing, towards a line, 
I P, perpendicular to the surface, to a definite extent, de- 
pending on the angle of the incident ray. The experi- 
ment may be varied by allowing the light to fall on the 
surface at various angles, when it can be shown that the 
angle formed by the ray refracted in the water varies in 
proportion to the angle of the incident ray, and that the 
angles formed are bound together by a regular law. 
Another fact may be observed, that the smaller the angle 
at which a ray of light strikes the surface of water, or, in 
fact, any transparent surface, the greater will be the pro- 
portion of light reflected at its surface. 
Refraction may be clearly studied by plunging a stick 
into a vessel of water: the stick will appear bent at the 
point where it enters the liquid, as in Fig. 4, thus giving 
the appearance as if the stick were lifted or bent upwards. 
Another very instructive experiment is to place a coin at 
the bottom of a vessel, and then, standing so that the 
coin is just hidden by its edge, to gradually fill the vessel 
with water ; the coin will appear to rise with the bottom 
of the vessel, and will become visible, as shown in Fig. 5. 
The amount of refraction varies with the medium em- 
ployed, and also with its temperature. The effect of 
different media can be clearly seen by passing a ray of 
Fic. 7.—Deviation of luminous rays by prisms. 
light into a vessel containing a liquid such as bi-sulphide 
of carbon, with a layer of water floating on the top. The 
ray will be seen to be bent on entering the water, and 
still more bent on passing from the water into the layer 
of bisulphide of carbon. 
We have now to see what takes place when a ray of 
light enters a piece of glass. We will take first the case 
of glass with parallel sides. The ray on entering the 
glass at the upper surface is refracted downwards, as in 
the case of water, and travels through the glass until it 
reaches the under surface. Here we have precisely the 
reverse condition holding—that is, the ray of light passes 
from a dense medium to a rarer one. The ray is re- 
fracted upwards or away from the perpendicular line, 
and thus will exactly neutralise the previous refraction, 
and the beam of light will come out in a direction parallel — 
to its original path, though not quite in the same straight 
line ; as shown in Fig. 6, the ray, instead of proceeding 
in the direction of S’, proceeds in the direction of S, 
