112 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



on the ro- tion by the doctrine of interference of the colours 

 tatory ac- o f Newton's rings received an important confir- 

 ^uartz Cation from an experiment of Arago's, which proved 

 them to arise from the mixture of the pencils 

 of light reflected at the two neighbouring sur- 

 faces. He pressed a lens of glass against a plate of 

 metal, in which case the central spot is white or 

 black when light polarized perpendicularly to the 

 plane of incidence is reflected at an angle greater or 

 less than the polarizing angle for glass ; and the rings 

 vanish altogether at the polarizing angle ; results 

 which have been found conformable to the undulatory 

 theory. 1 He also discovered the peculiarity of the 

 rays transmitted along the axis of a crystal of quartz. 

 These depolarize light, or produce colours similar to 

 those of crystallized plates, varying according to a 

 well-marked law with the thickness of the plate. 

 The most singular fact is, that by turning round the 

 analyzing plate, no position of neutrality is found, 

 but a series of colours similar to those of Newton's 

 scale succeed one another. Arago showed that this 

 effect is due to a rotatory motion of the plane of polar- 

 ization within the crystal. The rotation is greater 

 for the violet than the red ray ; this was shown by 

 M. Biot, who also discovered that in some specimens 

 the rotation takes place from right to left, in others 

 from left to right a peculiarity connected with cer- 

 tain crystallographic modifications, as was first 

 shown by Sir John Herschel. 



f.512.") MM. Seebeck and Biot discovered an analogous 



Rotation of property in oil of turpentine, and in various saccharine 

 the plane fluids, an observation which, in many cases, allows the 

 tion in substitution of an instantaneous optical, for an operose 

 fluids. chemical test. Fresnel has shown that the phenomena 

 of quartz may be represented on the supposition that 

 the rays traversing the axis consist of two rays circu- 

 larly polarized in opposite directions, and travelling 

 with different velocities; and Mr Airy succeeded in 

 calculating, by the aid of this fundamental hypothesis, 2 

 a number of most beautiful and complicated pheno- 

 mena, such, for example, as those which occur 

 when plates of right and left handed quartz are 

 superposed. Maccullagh has shown how the ge- 

 neration of elliptical or circular vibrations may be 

 deduced from the general equations of motion, but he 

 has not invented a mechanical theory to explain 

 them. This is a point of the very highest interest, 

 inasmuch as Dr Faraday has succeeded, for the first 

 time, in inducing artificially in a substance the power 

 of rotating the plane of polarization by the presen- 

 tation of it to the poles of a most powerful magnet. 3 

 (513.) Arago's experiments on the non-interference of 

 Retarda- rays of light oppositely polarized, being undertaken 

 tion of light j n conjunction with Fresnel, have been already re- 



e ' 



ferred to, arts. 488 and 506 ; but to Arago alone is in densa 

 due the ingenious idea of interposing a thin slip of media 

 mica or blown glass in the path of one of the inter- ^ 

 fering pencils, and observing the displacement of the 

 interference-bands, which is always towards the side 

 of the interposed slip, showing that the movement 

 of the wave has been slower within the denser me- 

 dium. 



Arago continued to attach great importance to the (514.) 

 obtaining of a still more direct proof of this fact, which Experi- 

 he considered as a crucial one between the rival hypo- ment 

 theses of Newton and Huygens. In his last years he ^"^ 

 had the satisfaction of witnessing the accomplishment C ault. 

 of it, with the result he anticipated, and by a method 

 which he had himself indicated. In 1838, he had al- 

 ready indicated the application of Mr Wheatstone's 

 beautiful invention of the revolving mirror, 4 as a means 

 of measuring intervals of time incredibly short, in order 

 to compare the velocity of light in air, and in a corre- 

 spondinglength ofwater. He even caused an apparatus 

 to be partly prepared, but we have seen that Arago's 

 forte was rather in suggesting than in completing re- 

 searches. After his increasing failure of sight ren- 

 dered it physically impossible thal^he should ever 

 realize his own idea, it was skilfully adopted by M. 

 Foucault (the author of the admirable experiment 

 with the pendulum, demonstrating the earth's mo- 

 tion 5 ), who, by an ingenious combination of fixed and 

 revolving mirrors, succeeded in 1850 in demonstrating 

 the retardation of light in a tube of water only 6^ 

 feet long, and with a velocity of rotation of the move- 

 able mirror not exceeding 200 turns in a second (a ra- 

 pidity four times less than had already been obtained 

 by Mr Wheatstone). The rotation was produced 

 by means of the Sirene of M. Cagniard de la Tour, 

 acting by steam. The velocity was thus raised to 

 1000 revolutions. It was afterwards, however, carried 

 by MM. Fizeau and Breguet to 2000 revolutions. It 

 will be understood, from the account of the method, 

 as applied to the measurement of the velocity of elec- 

 tricity, in another chapter, that the retardation is 

 shown by the displacement of the image of a minute 

 object seen through the water, relatively to the image 

 of the same object seen in air. If light moves faster 

 in water (as Newton imagined), the displacement of 

 the water-image will be (let us say) to the right ; but 

 if slower (as Huygens and Young believed), it will 

 be to the left. The calculated displacement, with 

 800 revolutions in a second, was '004 inch on the 

 first supposition, and "003 in the opposite direction 

 in the second, quantities easily visible with a high 

 magnifying power. The result, as has been stated, 

 confirmed Arago's original experiment of 1815 on 

 the displacement of the interference fringes. 



1 Sir W. Herschel first formed Newton's rings between glass and metal. Arago's experiment was reproduced (unknowingly) 

 by Mr Airy in 1831, who first explained it fully in the undulatory sense. Camb. Tram., vol. iii. 



2 With this addition, that rays inclined to the axis are elliptically polarized, and that with a greater ellipticity as the inclina- 

 tion increases. 



See the chapter on Electricity, 5. 



4 See Electricity, 6. 



5 See the chapter on Astronomy, Art. (258). 



