OPTICS. 



311 



fi ornaments. Mis labours were, however, followed 

 up by the researches of M. Arago and M. Biot, and, 

 at a more recent date, by Sir David Brewster. This 

 last mentioned philosopher, whose discoveries have 

 enriched various departments of the science, directed 

 his attention to the effect of heat arid compression on 

 polarization, and on the relation which the chemical 

 composition of bodies bears to their optical properties. 

 He also conducted many experiments, with a view of 

 determining the refractive powers of substances, and 

 obtained many valuable results, particularly with 

 regard to the gases. He invented several instru- 

 ments, among which may be more particularly men- 

 tioned the kaleidoscope. The science is much in- 

 debted also to the labours of M. M. Fregnel, Fraun- 

 hofer, Berard, Dr Morchini, Sir J. W. Herschel, and 

 others, who are still labouring in the field of dis- 

 covery. Our limits will not admit of a more extended 

 view of the history of optics. More minute parti- 

 culars will be found detailed in the dissertation by 

 Dr Thomson, accompanying this Encyclopedia. 



Definitions. By a ray of light is meant the motion 

 of a single particle; and Hs motion is represented by a 

 straight line. Any parcel of rays, passing from a point, 

 is called a pencil of rays. By a medium, is meant any 

 pellucid or transparent body, whicli suffers light to 

 pass through it ; thus water, air, and glass, are called 

 media Parallel rays are such as move always at 

 the same distance from each other. If rays continu- 

 ally recede from each other, they are said to diverge; 

 if they continually approach towards each other, they 

 are said to converge. The point at which converging 

 rays meet is called the focus ; the point towards 

 which they tend, but which they are prevented from 

 coining to, by some obstacle, is called the imaginary 

 focus. When rays, after passing through one me- 

 dium, on entering another medium, of different den- 

 sity, are bent out of their former course, and made 

 to change their direction, they are said to be refracted; 

 when they strike against a surface, and are sent back 

 again from the surface, they are said to be reflected. 

 A lens is glass ground into such a form as to collect 

 or disperse the rays of light which pass through it. 

 These are of different shapes, and from thence re- 

 ceive different names : a plano-convex lens has one 

 side flat, and the other convex ; a plano-concave lens 

 is flat on one side, and concave on the other ; a double 

 convex lens is convex on both sides ; a double concave 

 lens is concave on both sides ; a meniscus is convex 

 on one side, and concave on the other. A line pass- 

 ing through the centre of a lens is called its axis. 



Of Refraction. Although a ray of light will al- 

 ways move in the same straight line, when it is not 

 interrupted, yet every person knows, that when light 

 falls upon a drop of water, or a piece of glass, or a 

 bottle containing any fluid which allows the light to 

 pass, it does not reach the eye, or illuminate a piece 

 of paper placed behind those bodies, in the same 

 manner as before they were put in its way. This 

 obviously arises from the direction of the light being 

 changed, by some power which resides in the bodies. 

 The explanation of the law, or rule, by which this 

 change in the direction of a ray takes place, consti- 

 tutes that part of the science of optics called diop- 

 trics, from two Greek words, one of which signifies 

 through, and the other to see, because the bodies 

 which produce this change are those through which 

 we can see, or through which light passes. If the 

 rays of light, after passing through a medium, enter 

 another of a different density, perpendicular to its 

 surface, they proceed through this medium in their 

 original direction. But if they enter obliquely to the 

 s'irface of a medium, either denser or rarer than what 

 tlicy moved in before, they are made to change their 

 direction in passing through that medium. If the 



medium they enter be denser, tiiey move through it 

 in a direction nearer to the perpendicular drawn to 

 its surface. On the contrary, when light passes out 

 of a denser into a rarer medium, it moves in a direc- 

 tion farther from the perpendicular. This refraction 

 is greater or less, that is, the rays are more or less 

 bent, or turned aside from their course, as the second 

 medium through which they pass is more or less 

 dense than the first. To prove this in a satisfactory 

 way, take an upright empty vessel into a darkened 

 room, which admits but a single beam of light 

 obliquely through a hole in a window shutter. Let 

 the empty vessel stand on the floor, a few feet in ad- 

 vance of the window which admits the light, and let 

 it be so arranged, that as the beam of light descends 

 towards the floor, it just passes over the top of the 

 side of the vessel next the window, and strikes the 

 bottom on the side farthest from the window. Let 

 the spot where it falls be marked. Now, on filling 

 the vessel with water, the ray, instead of striking the 

 original spot, will fall considerably nearer the side 

 towards the window. And if we add a quantity of 

 salt to the vessel of water, so as to form a dense so- 

 lution, the point where the ray strikes the bottom 

 will move still nearer to the window. In like man- 

 ner, if we draw off the salt water, and supply its 

 place with alcohol, the beam of light will be still 

 more highly refracted ; and oil will refract yet more 

 than alcohol. In these experiments, if the room be 

 filled with dust, the rays will be rendered much more 

 visible. 



Thus in fig. l. plate LXIV. let A be the hole in 

 the window shutter admitting the ray of light, A C, 

 which falls obliquely upon the surface of the water 

 in the vessel, a section of which is marked by the 

 dark part of the figure. Were the vessel empty, 

 then the ray of light A C would proceed onwards in 

 the straight line A C B, and fall on the side of the 

 vessel at the point B. But when the vessel is filled 

 with water up to the level of C, the ray of light A 

 C will, on the moment of its entering the water, be 

 bended or refracted out of its original course, and 

 proceed in the direction C H, falling on the side 

 of the vessel at the point H, lower down than when 

 the vessel was empty. The line D C E is perpen- 

 dicular to the surface of the water in the vessel, so 

 that, as said above, the ray passing out of the rarer 

 medium, the air, into the denser, the water, is refract- 

 ed near to the perpendicular. Again, if a bright 

 object, such as a silver coin, be placed in the vessel 

 of water at H, a ray of light proceeding from it will 

 proceed first in the direction H C, but having arrived 

 at the surface of the water, -it will, on entering the 

 rarer medium, the air, take the direction C A, more 

 inclined from the perpendicular. By moving the 

 coin farther up to the point B, the ray will proceed 

 in the direction B C, but, after passing into the air, 

 will take a direction farther from the perpendicular 

 than the line B C A, and arrive at the circumference 

 of the circle somewhere below the point A, so that 

 the angle of incidence increases and decreases, as 

 the angle of refraction increases or decreases, but 

 not in equal proportion. But if we measure the 

 lines A F and H G, which are the sines of these 

 angles, we shall find that they bear a constant pro- 

 portion, whatever be the angle. Thus, if the ray 

 pass out of water into air, as we have supposed, and 

 the line H G be measured, as also the line A F, 

 then H G will be to A F as 1 is to T33, whatever be 

 the angle ; if the dense medium be glass, instead of 

 water, then II G will be to A F in the constant pro- 

 portion of 1 to 1 -5, whatever be the angle. Now, 

 the sine of the angle of incidence being always re- 

 garded as one, the relative refractive powers of water 

 and glass will be as the number 1-33 to 1-5, and 



