464 



NA TURE 



{April 16, 1874 



POLARISATION OF LIGHT* 

 VII. 



AMONG the phenomena of polarised light which may 

 be observed either with a Nicol's prism or even with 

 the naked eye, one of the most curious, and perhaps not yet 

 fully explained, is that of Haidinger's brushes. If the eye 

 receives a beam of polarised light a pale yellow patch in 

 the form of an hour-glass, the axis of which is perpen- 

 dicular to the plane of vibration, is perceived. On either 

 side of the neck of the figure two protuberances of a 

 violet tint are also seen to extend. After a little practice 

 these figures or "brushes" may readily be observed. If 

 the day be cloudy a Nicol must be used and directed to 

 a tolerably bright c'oud. The brushes are better defin d 

 in one position than in others ; but if the Nicol be turned 

 round, the brushes will be seen to revolve with it. If on 

 a clear day we look in a direction at 90° from that of the 

 sun, where the skylight is most completely polarised, the 

 brushes may be seen with a naked eye. Jamin ha- s;ig- 

 gested in explanation of this phenomena that the sub- 

 stances of the eye act like a pile of glass plates, or rather 

 spheres, which affect in different degrees (i) the rays of 

 the same colour whose vibrations are differently inclined 

 to the plane of incidence, and (2) the rays of different 

 colours whose vibrations are similarly inclined. This will 

 cause one colour to predominate in a general direction 

 parallel, and its complementary to predominate in a plane 

 perpendicular, to that of vibration. Hehnholtz, however, 

 connects the phenomenon with some double refraction 

 due to the yellow spot in the eye, with the area of which 

 that of the brushes is coincident. 



It was explained above that in Iceland spar there is a 

 particular direction, viz. that of the line joining the two 

 opposite obtuse angles of the natural crystal, in which 

 there is no double refraction, and in which all rays travel 

 with the same velocity. This direction (that is to say, 

 this line and all lines parallel to it) bears the name of the 

 optic axis. There are many other cryr'r.U having the 

 same property in one and only one direction, in other 

 words having a single optic axis. There is, moreover, 

 another class of crystals having two such axes. Crystals 

 of the first class or uni-axal crystals are again divided 

 into two groups, viz. positive, in which the extraordinary 

 ray is more refracted than the ordinary, and negative, in 

 which the ordinary ray is the more refracted. It will be 

 remembered that the ray which travels slowest is the 

 most refracted. Among the former may be mentioned 



Uni-axai> Crystals. 



Positive: 



Apophjllitc. 



Boracite. 



Ditopaz. 



Hydrate of magnesia. 



Hyposulphate of lead. 



Ice. 



Quartz. 



Red Silver. 



Siannite. 



Superacetate of copper and 



lime. 

 .Sulphate of potash and 



iron. 

 Tungstate of zinc. 

 Zircon. 



Xi'tra/ivc. 



Apatite. 



Arseniate of copper. 



Arseniate of lead. 



Arseniate of potash. 



Beryl. 



Carbonate of lime and 



magnesia. 

 Carbonate of lime andiron. 

 Chloride of calcium. 

 Chloride of strontium. 

 Cinnabar. 

 Corundum. 

 Emerald. 



HonLy stone. 



Idocrase. 



Mellite. 



Mica. 



Molybdate of lead. 



Nepheline. 



Octaedrite. 



Phosphate of lime. 



Phosphate of lead. 



Rubellite. 



Ruby. 



Sapphire. 



Continued from p. 3S6. 



Crystals are usually divided into six systems, in each of 

 which there is a fundamental aad a variety of derived 

 forms. The fundamental form of each system is based 

 upon the number, magnitude, and inclination of the 

 crystallographic axes or lines drawn through a point in 

 the interior of the crystal, and terminating in its angles. 

 The optic axes do not of necessity coincide with any of 

 these. 



(i.) The regular system, which is based upon a system 

 of three equal rectangular axes. Any form derived from 

 this will be perfectly symmetrical with reference to the 

 three axes, and will present no distinguishing feature in 

 relation to any of them. Crystals belonging to this 

 system have no optic axis, nor any doubly refracting 

 property. 



(2.) The quadratic system, based upon a system ol 

 three rectangular axes, whereof two are equal, but the 

 third greater or less than the other two. Crystals be- 

 longing to this system have one optic axis coinciding 

 with the last-mentioned crystallographic axis. 



(3.) The hexagonal system, having three equal axes 

 lying in one plane inclined at 60'^ to one another, and a 

 fourth axis at right angles to the other three. Crystals of 

 this system have one optic axis coinciding with the fourth 

 crystallographic axis. Iceland spar belongs to one of the 

 derived forms of this system. 



(4 ) The rhombic system, having three rectangular but 

 unequal axes. 



(5.) The monochnic system, which differs from the 

 rhombic in this, that one of the three axes is oblique 

 to the other two, which are still rectangular to one 

 another. 



(6.) The triclinic system in which all the axes are 

 obhque. 



All crystals belonging to the last three systems have 

 two optic axes. In the rhombic system they lie in a plane 

 containing two of the three crystallographic axes ; in the 

 monoclinic, they lie either in the plane containing the 

 oblique axes, or in a p!ane at right angles thereto. In 

 the trichnic no assignable relation between the optic and 

 the crystallographic axes has been determmed. 



The phenomena of colours and their variations hitherto 

 described have been produced by a beam of light, all the 

 rays of which were parallel in their passage through the 

 crystal or other substance under examination. There is, 

 however, another class of phenomena due to the trans- 

 mission of a convergent or divergent beam of polarised 

 light, which we now proceed to consider. 



It was shown above that the retardation due to any 

 doubly refracting crystal, and consequently the colour 

 produced by it is dependent on the thickness; and that 

 with a crystal of constantly increasing thickness, the 

 colours go through a complete cycle, and then begin 

 again. Suppose then a divergent beam to fall perpen- 

 dicularly upon a uni-axal crystal plate cut at right angles 

 to the optic axis ; the central rays will fall perpendicularly 

 to the surface ; but the rays which form conical shells 

 about that central ray will fall obliquely. The rays 

 forming each shell will fall with the same degree of 

 obliquity on different sides of the central ray, those form- 

 ing the outer shells having greater obliquity than the 

 inner. Now the more obliquely any ray falls upon the 

 surface the greater will be the thickness of the crystal 

 which it traverses ; and this will still be the case even 

 though it suffers refraction, or bending towards the per- 

 pendicular on entering the crystal. Each incident cone 

 of rays will consequently still form a cone when re- 

 fracted within the crystal, although less divergent than 

 at incidence, in its passage through the plate ; and 

 the successive refracted cones will be more and more 

 oblique, as were the incident cones, but in a less degree, 

 as we pass from the more central to the more external 

 members of the assemblage forming the beam of light. 



LetABCD(Fig. 21) represent the crystal plate, O Pthe 



