- OPTICS. 117 
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passing from a rarer into a denser medium, it will be nv when the ray tra- 
verses the same media in the reverse direction. As this minimum angle 
of incidence = 0°, that is, when the ray falls perpendicularly to the 
coinciding surfaces of the media, the angle of refraction must, in that case, 
be 0°, or the ray will pursue its course unbroken. The greatest value of 
1 
the angle of incidence will be 90°; and as sin. 90° = 1,7 == (when 
1 
ris the angle of refraction), or sin. r = >. This value of 2 is called the 
limiting angle. | 
For the media air and water, n=4; thus ; = #=0.75 = sin. 48° 35’, 
and this value is the limiting angle in this instance. Then a ray of light, 
passing from air into water, cannot have an angle of refraction greater than 
48° 35'; if a ray pass at this angle from water into air, its refraction will 
amount to 90°, or the refracted ray will be parallel to the surface of 
refraction. All rays, then, proceeding from water to air, which strike the 
refracting medium at an angle less than the limiting angle, will not pass 
out. but will be entirely reflected back again, as illustrated in fig. 78, where 
the ray loses nothing of its original intensity by reflection. Fig. 26 
represents a particular instance of such total reflection. Dip an empty 
glass tube, melted together at the bottom, into a vessel filled with water. 
By giving it a position something like that in the figure, and looking at the 
tube from above, it will appear as if filled with mercury. By pouring 
water into the tube, the metallic lustre will vanish as far as the water 
reaches. The phenomenon is easy of explanation, as the rays coming from 
a strike the tube at such an angle as not to be capable of entering into the 
air of the tube; consequently they are reflected. This reflection must, 
however, cease as soon as water is poured into the tube. : 
The amount of deviation, or the angle of deviation, may always be 
obtained by subtracting the angle of refraction from the angle of incidence. 
This deviation does not increase proportionally, as it increases with the 
increase of the angle of incidence much more rapidly than of the angle 
of refraction. 
A prism, in Optics, is a transparent medium, bounded by two inclined 
sides. ‘The line in which these two sides intersect, is called the refracting 
edge, and the side opposite to this the base. The angle of the two surfaces 
is called the refracting angle; the intersection of the prism, by a plane at 
right angles to the edge, is called the principal section. The three-sided 
prism is generally employed, bounded by three rectangular parallelograms 
( fig. 79); the principal section of such a prism i§ a triangle. In optical 
experiments, the prism is usually fastened upon a small brass stand 
(fig. 27). The rod, ¢, may be moved up and down in the tube in which it 
is placed, and the prism may be placed in any direction required, by means 
of a hinge at g. If the prism be fixed with the refracting edge uppermost, 
all objects seen through it will appear considerably displaced and raised 
291 
