OPHTHALMOSCOPE AND LARYNGOSCOPE.

991

to concentrate light upon an object for the purpose of rendering it more distinctly visible.

The ophthalmoscope is a small concave mirror, with a small hole in its centre, through which the observer looks from behind, while he

directs a beam of reflected light from a lamp into the pupil of the patient's eye. In this way (with the help sometimes of a lens) the retina can be rendered visible, and can be minutely examined.

The laryngoscope consists of two mirrors. One is a small plane mirror, with a handle attached, at an angle of about 45° to its plane. This small mirror is held at the back of the patient's mouth, so that the observer, looking into it, is able by reflection to see down the patient's throat, the necessary illumination being supplied by a concave mirror, strapped to the observer's forehead, by means of which the light from a lamp is reflected upon the plane mirror, which again reflects it down the throat.

Some additions to this chapter will be found at page 1086.

CHAPTER LXIX.

REFRACTION.

981. Refraction. When a ray of light passes from one transparent medium to another, it undergoes a change of direction at the surface

of separation, so that its course in the second medium makes an angle with its course in the first. This changing of direction is called refraction.

The phenomenon can be exhibited by admitting a beam of the sun's rays into a dark room, and receiving it on the surface of water contained in a rectangular glass vessel (Fig. 687). The path of the beam will be easily traced by its illumination of the small solid particles which lie in its course.

The following experiment is a well-known illustration of refraction:-A coin mn (Fig. 688) is laid at the bottom of a vessel with

opaque sides, and a spectator places himself so that the coin is just hidden from him by the side of the vessel; that is to say, so that the line m A in the figure passes just above his eye. Let water now be poured into the vessel, care being taken not to displace the coin. The bottom of the vessel will appear to rise, and the coin will come into sight. Hence a pencil of rays from m must have entered the spectator's eye. The pencil in fact undergoes a sudden bend at the surface of the water, and thus reaches the eye by a crooked course,

LAWS OF REFRACTION.

993

in which the obstacle A is evaded. If the part of the pencil in air be produced backwards, its rays will approximately meet in a point m', which is therefore the image of m. Its position is not correctly indicated in the figure, being placed too much to the left (§ 990).

The broken appearance presented by a stick (Fig. 689) when partly immersed in water in an oblique position, is similarly ex

Z Z'

P

A

Fig. 688.-Experiment of Coin in Basin.

plained, the part beneath the water being lifted up by refraction.

982. Refractive Powers of Different Media.-In the experiments of the coin and stick, the rays, in leaving the water, are bent away from the normals ZIN, Z'I'N' at the points of emergence; in the experiment first described (Fig. 687), on the other hand, the rays, in passing from air into water, are bent nearer to the normal. In every case the path which the rays pursue in going is the same as they would pursue in returning; and of the two media concerned, that in which the ray makes the smaller angle with the normal is said to have greater refractive power than the other, or to be more highly refracting.

Fig. 689.-Appearance of Stick in Water.

Liquids have greater refractive power than gases, and as a general rule (subject to some exceptions in the comparison of dissimilar substances) the denser of two substances has the greater refracting power. Hence it has become customary, in enunciating some of the laws of optics, to speak of the denser medium and the rarer medium, when the more correct designations would be more refractive and less refractive.

983. Laws of Refraction.-The quantitative law of refraction was not discovered till quite modern times. It was first stated by Snell, a Dutch philosopher, and was made more generally known by Descartes, who has often been called its discoverer.

Let RI (Fig. 690) be a ray incident at I on the surface of separation of two media, and let IS be the course of the ray after refraction. Then the angles which RI and IS make with the normal are called the angle of incidence and the angle of refraction respec

tively; and the first law of refraction is that these angles lie in the same plane, or the plane of refraction is the same as the plane of incidence.

The law which connects the magnitudes of these two angles, and which was discovered by Snell, can only be stated either by reference to a geometrical construction, or by employing the language of trigonometry. Describe a circle about the point of incidence I as centre, and drop perpendiculars, from the points where it cuts the rays, on the normal. The law is that these perpendiculars R' P', SP, will have a constant ratio; or the sines of the angles of incidence and refraction are in a constant ratio. It is often referred to as the law of sines.

The angle by which a ray is turned out of its original course in undergoing refraction is called its deviation. It is zero if the incident ray is normal, and always increases with the angle of incidence.

984. Verification of the Law of Sines.-These laws can be verified by means of the apparatus represented in Fig. 691, which is very similar to that employed by Descartes. It has a vertical divided circle, to the front of which is attached a cylindrical vessel, half-filled with water or some other transparent liquid. The surface of the liquid must pass exactly through the centre of the circle. I is a movable mirror for directing a reflected beam of solar light on the centre O. The beam must be directed centrally through a short tube attached to the mirror, and to facilitate this adjustment the tube is furnished with a diaphragm with a hole in its centre. The arm Oa is movable about the centre of the circle, and carries a vernier for measuring the angle of incidence. The ray undergoes refraction at O; and the angle of refraction is measured by means of a second arm OR, which is to be moved into such a position that the diaphragm of its tube receives the beam centrally. No refraction

occurs at emergence, since the emergent beam is normal to the sur

faces of the liquid and glass; the position of the arm accordingly indicates the direction of the refracted ray. The angles of incidence and refraction can be read off at the verniers carried by the two arms; and the ratio of their sines will be found constant. The sines can also be directly measured by employing sliding-scales as indicated in the figure, the readings being taken at the extremity of each arm.

It would be easy

to make a beam of light enter at the lower side of the apparatus, in

a radial direction; and it would be

found that the ratio of the sines was precisely the same as when the light entered from above. This is merely an instance of the general law, that the course of a returning ray is the same as that of a direct ray.

985. Airy's Apparatus.-The following apparatus for the same purpose was invented, many years ago, by the present astronomer royal. B' is a slider travelling up and down a vertical stem. A C' and B C are two rods pivoted on a fixed point B of the vertical stem. C'B' and CB' are two other rods jointed to

B

Fig. 692.-Airy's Apparatus.

the former at C' and C, and pivoted at their lower ends on the centre