iEi'T. 4, 1885.] 



♦ KNOW^LEDGE ♦ 



is the better, and ^^f :-.Il I'vcuis ilir length of one of its 

 edges must be at least ::,n iml]), :iuil making a circular 

 dot, W, on a sbeet of i':t|i<r, jit.cc (.ne face of tlie crystal 

 on it. Then, to an eye j^hK-cd :il K, the dot will be seen 

 double, as W, W, and if we turn tlie crystal round, the 

 same side always touching the jiaper, we shall see the dot 

 W remain apparently stationary, and the dot W^ describe a 

 seeining orbit rouud it. Now we have several times 

 insisted, in these papers, on the fact that light goes and 

 returns by the same route, and hence the reader will be 

 o expect that if instead of viewing a spot from 

 e place a .source of light, the beam Ee will be 

 split on entering the crystal, and its image be seen as 

 two to an eye placed on the other side of what was the 

 base of our crystal in our fir.st experiment. Moreover, 

 when we come to investigate the bending of this double 

 ray, we shall tind tluit while eW follows the ordinary law 

 of sines (vol. vi., p. 32), and lies, of course, in the plane of 

 incidence, eW> does nothing at all of the sort, but 

 follows some new and remarkable law. Hence eW is 

 called the ordinary ray, and eW the extraordinary ray. 

 1 L may not, perlia}:s, be wholly out of jilace to add here 

 that when the ray Ee is incident perpendicularly on the 

 face A B C D, as in other cases, the ordinary ray 

 passes straight through the crystal, and is not refracted 

 at all, while the extraordinary ray, under these circum- 

 stances, has an angle of refraction of 6° 12', and is bent 

 on one side. Lastly, we may note that a ray of light 

 passing in the direction A X suffers no double refraction 

 whatever. 





Fig. 47. 



Now we have gone into all this seemingly irrelevant 

 detail, because in splitting the incident beam of light 

 into two our crystal has done something else : in point of 

 fact it has separated our original beam A B C D (Fig. 44) 

 into two, A B and CD, each having different properties 

 on different sides ; or rather, A B has the same properties 

 at its sides A and B that C D has at its sides C and D. 

 In other words, the vibrations of which the beam of 

 common light A B C D is made up have been split into 

 two separate sets A B and C D at right angles to each 

 other. Each, then, of these separate beams is said to be 

 polarised, and the planes passing through the lines A B 

 and C D are called the planes of polarisation of the beams 

 respectively. From this we may infer, what we shall 

 find cxiicrinicntally to be the fact, that a beam of common 

 light A B C D may be regarded as made up of two beams 

 of polarised light with their [.lanes of polarisation at 



if \ 



L- pl:„ 



a uf ,, 



as either of the separate beams composing it. We have 

 only to employ a second rhomb of Iceland spai' to show 



For this purpose it is better t.) nimmt ouv two vIh.ihIis 

 of spar, as shown in Fig. 47, «1iriv '1' :ui.l '1" sl,ou Iw,. 



means of cork rings. Both ends cf I he tiil.rs ;iiv r.ivcnd 

 by brass di.scs d d d' d', perrnratcl ,-enlrally with lu.lcs. 

 Placing our tubes, then, in a liori/.nntal jinsition, and 



letting a beam of parallel solar rays /• o pass through the 

 left hand opening, it will be seen, from what has preceded, 

 that only the ordinary ray r o will emerge through the 

 opposite hole, the extraordinary ray r e being refracted 

 up to e, whence, of course, its egress is prevented by the 

 solid brass plate. If now the two rhombs are so situated 

 that their principal planes passing through r o and the 

 optic axis lie in the same plane (say that of the paper), 

 the ordinarily refracted ray r o will pass similarly 

 through the second crystal, as at / o'. If, though, keeping 

 the first tube fixed, we rotate the second one about its 

 axis, we shall find that double refraction will take place, 

 and that an extraordinary as well as an ordinarily 

 refracted spot of light will become visible. This extra- 

 ordinarily refracted ray, at first dim, becomes brighter 

 as the crystal is rotated, the ordinary ray diminishing 

 corrcs|HiiiiliiiL:l) in illumination until the angle between 

 the two 1 liiiiiial j'lanes of cleavage of the rhombs = 45°. 

 As the rutatii'ji continues, the ordinary ray continues 

 to fade out and the extraordinary ray to become 

 stronger until, when the principal planes are square to 

 each other, the ordinary ray vanishes, and the extra- 

 ordinary one remains of the full strength of the 

 original beam o r. Light may also conveniently be 

 polarised by the aid of slices of the mineral tourmaline, 

 cut parallel to the optic axis of the crystal. Tourmaline 

 of moderate thickness entirely extinguishes the ordinary 

 ray, but transmits the extraordinary ray (which vibrates 

 parallel to its crystalline axis) perfectly. Tourmaline is 

 of all sorts of colours, some splendid specimens from 

 Devonshire being of so dark a brown as to appear almost 

 black. If slices of suflBcient thinness can be cut from 



Fig. 48. 



these dark crystals, they are generally effective, but this 

 is no easy matter. Those usually found in commerce are 

 brown, green or red. They, of course, colour the light 

 to a certain extent if they are of a dark tint or cut into 

 thick slices, but this is no great practical disadvantage. 

 Two such plates mounted in cells which can be rotated, 

 the cells being borne at the ends of a twisted wire, are 

 sold by opticians uu.h i- tlic name of " Teiirumliiie teii-s." 

 Any object re(|uire(l f,i \h- viewed \>y ] el:. vised li^-ht mav 

 be nipped between the u ire I'in-s in wliieli the cell's 

 rotate. If now we jilacc our tw.. phil,- in s;ic!i a 



of our plates as being that of a (jratimj, with its parallel 



