POLARISATION OF LIGHT. 



9 



at right angles to "one another, which 

 will be discovered by turning the reflect- 

 ing surface round the polarised ray. 



This beautiful property of light, in 

 virtue of which it is polarised by reflexion, 

 and refuses to be again reflected under 

 the circumstances above described, was 

 discovered, in 1810, by M. Malus, a 

 French philosopher . of distinguished 

 eminence. 



In continuing his researches, Malus 

 found that black marble, ebony, and 

 other opaque bodies, polarised the light 

 by reflexion like transparent ones ; and 

 that when the light RA was incident on 

 A at an angle below or above 55, only 

 a part of the reflected ray was po- 

 larised ; and that the light which fell 

 upon the second surface of the glass 

 plate was polarised at the same ^time 

 with that which fell upon the first "sur- 

 face. He found the angle of incidence 

 upon water, at which it polarised the 

 light most completely, to be 52 45', and 

 the angle for glass to be 55 ; and he 

 concluded that the property by which 

 bodies polarised light was independent 

 of the other modes of action which they 

 exert upon light. 



The experiment represented in Jig. 12 

 is susceptible, as Dr. Brewster has 

 shown, of a singular and pleasing varia- 

 tion. If, in the position shown in the 

 figure, when the ray AC is not reflected, 

 and the body from which it proceeds 

 therefore not seen to an eye at E, we 

 breathe gently upon the glass E, the ray 

 C E will be, as it were, revived, and the 

 candle or body from which RA proceeds 

 will become instantly visible. The rea- 

 son of this is,], that a thin film of water 

 is deposited up*on the glass by breathing ; 

 and as water polarises light at an angle 

 of 52 45', the glass C should have been 

 inclined at an angle of 52 45' to AC, in 

 order to be incapable of reflecting the 

 polarised ray ; but as it is inclined at an 

 angle of 56, it has the power of reflect- 

 ing a portion of A C. 



If we now place the glass C at an an- 

 gle of 52 45' to AC, then it will reflect 

 a portion of the polarised ray to the eye 

 at E ; but if we breathe upon the glass 

 C, the reflected light will disappear, 

 because the reflecting surface is now 

 water, and is placed at an angle of 

 52 45', the polarising angle for water. 

 If we, therefore, place beside each other 

 two sets of reflectors, arranged as above 

 described, we may, by breathing upon 

 two adjacent plates of glass, exhibit the 



paradoxical phenomenon of recovering 

 and extinguishing a luminous image by 

 the same breath. 



While repeating the experiments of 

 Malus, Dr. Brewster measured the po- 

 larising angles of a great number of 

 transparent bodies, and found, from a 

 careful comparison of them, that they 

 led to the following simple law : 



The index of refraction for any trans- 

 parent body is the tangent of its angle 

 of polarisation. 



The following are" the observations by 

 which this law is confirmed. 



Observed Polarising T 

 angle. 



. 45 or 4 7 

 . 53 14'* 

 . 54 50 



. 56 " 

 56 



56 

 57 



Air . 



Water . 



Fluor spar . 



Obsidian 



Sulphate of lime 



Crown glass . 



Rock crystal 



Sulphate of barytes 57 



58 



58 

 58 

 59 

 58 

 58 

 60 

 63 

 64 

 64 

 68 

 67 

 67 

 67 



Topaz . 



Mother of pearl 



Iceland spar$ 



Spinelle ruby 



Zircon . 



Glass of antimony 



Sulphur 



Diamond 



Chromate of lead 



3 



28t 



12 



22 



471 



f 



29J 



401 

 OJ 



47 



51 



16 

 8 



45 



10 

 21 



13 f 

 Oj 



48 



Calculated Pola 

 rising angle. 



450'32" 

 53 11' 

 55] f 9 

 56, [6 

 56 45 

 56 45 

 56 58 



58 33 



58 34 



58 50 



58 51 



60 25 



63 



64 30 

 63 45 



68 1 



68 3 



If the original beam of light R A has 

 considerable intensity, it will be observed 

 that the reflected pencil C E does not 

 wholly vanish, and that the remaining 

 portion is coloured. This effect is finely 

 seen when we use oil of cassia, which 

 has a great dispersive power, or diamond 

 or chromate of lead. With glass it is of 

 a purple colour, and with oil of cassia 

 it is a fine blue; these colours varying 

 according as the angle of reflexion is 

 above or below the polarising angle. 

 This unpolarised light Dr. Brewster 

 ascribed to the circumstance, that as the 

 different rays had in every substance 

 different indices of refraction, they 

 would have also by the general law dif- 



* This is a mean of four observations by M. 

 Malus, M. Arago, M. Biot, and Dr. Brewster. 



t Mean of six observations. 



t This and other crystals with powerful double 

 refraction give different polarising angles in dif- 

 ferent azimuths. 



