UHDUL \T"K V FORCES. LIGHT. 



[POLARISATION, 



ixuil extraordinary rav* iu a i medium ..f thu 



kind wo are. now dealing with, differ from each other; 



tli.it of ti rnj to tliu 



aiiglo it makes with the axis of the crystal, wliiUt i!i..t 



aslant. Tint larger tin- angle which 



nlinary niy makes with the axis of tin- crystal. 



tin- u-i cater is i : ami its relation U> that of tin' 



<iriliit;iry ray is dependent ou the positive or negative 



character of thu crystal. 



POLARISATION OF LIGHT. 



PKHHAFS there are no ]>lieiiomna of which it is more 

 difficult to convey a ]M,pular description, than those 

 connected with Uie polarisation of light Wo Hhall, how- 

 ever, endeavour to meet the difficulty by affording simple 

 means whereby a ray of light may be polarised; give 



ions for constructing apparatus called polariscopes ; 

 ami after thus conveying a general idea of the effects of 

 polarisation, which the student may easily become ac- 

 quainted with, we shall go more deeply into the fact* 

 and principles of tliis interesting branch of physical 

 science. 



The .-indent will naturally ask, " Wliat do you mean 

 by polarised light ?" And we will endeavour to answer 

 the question, by pointing out certain effects which occur 

 when a ray of common light is reflected or refracted at 

 certain angles. 



When speaking of the reflection of light,* we stated, 

 that a ray, incident on any reflecting surface, was re- 

 flected at an angle equal to that of incidence. If, how- 

 ever, a ray of light fall ou a plate of glass (which should 

 be blackened on the bock), placed in such a position that 

 the my is incident at an angle of about 57, then the 

 r.iy is divided into two parts, one of which is reflected, 

 and the other is absorbed. If the reflected ray be received 

 by another glass plate, placed so that the ray reflected 

 from the tint is received at an angle of 57 on the second 



. the ray will be polarised ; and it has thus acquired 



in properties greatly diffaing from those of ordinary 

 white light. The student will understand this better by 

 constructing the simple apparatus represented in the 

 annexed engraving, which will afford him an elementary 

 |x>lariscope, or instrument by which rays of polarised 

 light may be obtained. 



KIR. 71. 



ft 



Fig. 71 illastrates the instrument to wliich we refer. 

 To construct it obtain two tin tubes, each six inches 

 long, and about two indies in diameter: and one of these 

 should be so much narrower than the other that it may 

 slide easily and yet firmly therein. The insides of the 

 tubes should be blackened by coating them with a mix- 

 ture of lampblaek and size. This will prevent any re- 

 flection of light internally, which would interfere with 

 the desired result. At one end of each tube a piece of 

 plate glass, blackened also at the back, must be pi 

 ami each of these is fixed at an anglu of :;:; . so th.it a 

 >f incident light may fall, at an angle of 57, on the. 

 surface of either. 



If the eye of the spectator be placed as at c in the 

 engraving, he will perceive tin- of the ray from 



a; but if the glass, a, be turned round to the extent of a 

 quarter of a circle that is, 90 he will find that the 

 reflection of light from it is almost entirely lost. On 

 turning " round to one-half of the circle, or 180, so that 

 and li shall lie ill entirely opposite positions, it will bo 

 observed that thu light reflected from <i will again ap 

 If the glass bo further moved so as to have passed 

 through three-quarters of the circle, the light will be 

 gain cut oil'. On continuing the motion of a until 

 restored to its primary condition, the reflected ray will 



IM ante, p. 4i . 



again appear; and thus we observe, at each quarter of 



tlie i '< or lo.%s of the ray incident ou a, as 



an the reflected surface, I. Now this only 



happens w hen each of these glasses is placed at a certain 



, as already remarked ; and such is therefore called 



the /;,!, ,'.-,;,,.; .;,,.//, of the substance employed for that 



purpose. It is at this angle that the greatest po!.. 



' takes place, although such also occurs at other 

 angles, but, of course, to a diminished extent. 



Another nuxle of producing similar ell'ects, is that of 

 lining two plates of a mineral called tourmaline, which 

 may be obtained of the optical instrument-makers. Or- 

 dinary light as of the sun, a candle, or a gas flame is 



. seen through one of these plates, or through two 

 if placed over each other in certain positions. If, how - 

 one plate be held be-fore any source of common 

 light, and the oilier bo gradually rc\ohvd through a 

 complete circle in front of it, and next to the eye of the 

 observer, it will be noticed, that at every quaitcr of .1 

 circle, or 90, the light will IHJ alternately exhibited or 

 extinguished, just as we slated would be found in using 

 the simple apparatus already described. There is how- 

 ever, this difference between the two processes the first 

 produces the result of polarisation by reflection, and the 

 latter by the Ira of the rays thro ugh a i enacting 



substance. The student will thus see, that what is called 

 polarisation, is equally effected on ordinary light by re- 

 flection and refraction. 



Having thus shown how light may be polarised, we 

 proceed to describe the nature of the effect produced ; 

 and this, perhaps, can bo best done by pointing out the 

 difference which exists between a polarised and a non- 

 polarised ray. An onl'mx, ;/ ray will be equally trans- 

 mitted and reflected through or from the body on wliich 

 it may fall ; whilst a polarised ray is not thus indifferent 

 to the physical nature of such body ; and, moreover, will 

 only pass in such directions through it, or be reflected 

 from its surface, at certain angles varying for each mate- 

 rial employed. Hence, in polarisation, a ray of ordinary 

 light acquires such properties as connect it essential with 

 the phyMcal nature of the substances on which it m.iv 

 fall, or pass through ; and this is evidenced from the fact, 

 that it will not pass through some substances at certain 

 angles; that it will similarly not be reflected at some 

 angles of incidence ; and lastly, its relationship to doubly 

 refractive structures is eminently different to that it 

 maintained previous to polarisation. On the latter pro- 

 perty we shall have to enlarge more fully hereafter. 



A variety of illustrations have In -en otiered, in scientific 

 works, for the purpose of familiarising the eye and the 

 mind of the student with these facts. One of the sim- 

 plest of these is represented in the annexed illustration. 

 (See Fig. 72). 



Ki. 72. 



In Fig. 72, let a b represent a piece of cardboard, 

 so as to form a cross, with another piece, < d \ and 

 at a flat piece of card, through wliich 

 parallel openings have been made from one side to the 

 other. Now, supposing either end of d b c d be presented 

 to,'/, only that portion parallel to the openings of ef 

 could lie introduced; the other part of the cross, being at 

 right angles to these openings, could not of course pass 

 through. Now, in a similar manner doea the plate of 

 tourmaline act on the ray of light; and in the above we 

 have illustrated the I round the 



tourmaline plates namely, that of the rays being alter- 

 nately transmitted or extinguished. Our remarks and 

 illustrations are, for the present, entirely con lined to the 

 leration of wliat is called /I/UK, liylit ; or, 



ill other words, we refer to the absorption or tnniRini.ssiou 

 of light, solely in connection with imaginary rectilinear 

 planes, in or about the bodies employed for our purpose. 



