POLARISATION OF LIGHT. 



19 



so as to reflect the brightest part of the 

 sky, take a thin film or slice D G E F, 

 oi sulphate of lime (or mica, if sulphate 

 of lime cannot be had) between the 20th 

 anil 60th part of an inch in thickness, and 

 hold it, as shown in the figure, between 

 the polarising and the analysing plates. 

 When, previous to the interposition of 

 the crystallised plate, the eye E, looked 

 into C", it saw only the dark spot above- 

 mentioned ; but it will now observe the 

 whole surface of the polarizing plates at 

 A, covered with colours of the richest and 

 most varied hues, and following one 

 another, according as the sulphate of 

 lime is more or less inclined, or accord- 

 ing as the light passes through thicker 

 or thinner portions of the film. If the 

 plate is equally thick, which with a little 

 care may be effected, and if it is held 

 perpendicular to the polarised light, there 

 will be found two lines D E, F Gr, which 

 have the property, that when either of 

 them is parallel or perpendicular to the 

 plane of primitive polarisation RAG, 

 or to the plane ACE, no colours are 

 seen, and the black spot appears exactly 

 as if the film were not interposed. These 

 two lines may be called the neutral 

 axes of the crystallised i film. If the 

 film D G E F is turned round in its 

 own plane, there will be found in all 

 other positions the phenomenon of a 

 single colour ; but this colour will be 

 most brilliant, when either of the lines 

 a b, cd, perpendicular to one another, 

 and each inclined 45 to the neutral 

 axes, is in the plane of primitive polari- 

 sation RAC, the brilliancy or intensity 

 of the colour gradually diminishing from 

 the position of no colour, to the position 

 where the colour is a maximum. The 

 two lines a b, cd, may be called the de- 

 polarising axes of the film. 



If we suppose the plates A and C, to 

 be fixed, and the film DGEF, to re- 

 volve round the ray A C, from a posi- 

 tion where no colour is seen, it will 

 then be found, that the colour which 

 we may suppose red, is a maximum at 

 the azimuths, or angles of revolution, of 

 45, 135, 225, and 315, while it dis- 

 appears altogether at azimuths of 0, 90, 

 180 and 270. If we now suppose the film 

 DGEF, to be fixed in any of the posi- 

 tions 45, &c. or where it produces the 

 brightest red, and if we cause the ana- 

 lysing plate C, to revolve round the 

 ray AC, its inclination to AC remain- 

 ing invariable, we shall observe the fol- 

 lowing phenomena. The brightest red 

 being visible at 0, or where the plate G 



begins to move, the brightness of this 

 colour will gradually diminish till C has 

 turned round 45, when the red colour 

 will disappear. Beyond 45, a faint green 

 will appear, and will gradually increase 

 in intensity till it reaches its maximum 

 brightness at 90. Beyond 90 the 

 green becomes paler and paler, till it dis- 

 appears at 135, where the red again 

 comes in, and reaches its maximum 

 brightness at 1 80: the very'same changes 

 are repeated between 180 and 360 or 

 0. Hence it follows, that when only 

 the film of sulphate of lime revolves, a 

 single colour merely is seen ; while, when 

 only the plate C moves, two colours 

 are seen during its revolution. 



By repeating the above experiment 

 with films of sulphate of lime that give 

 different colours, it will be found that 

 the two colours are always complement- 

 ary to each, or that the two together 

 make up white light. This curious pro- 

 perty may be ocularly demonstrated 

 by the following experiment. Instead of 

 analysing the light transmitted by the 

 sulphate of lime by the plate C, substi- 

 tute a prism of calcareous spar, that 

 gives two images, and when the plane of 

 the principal section of the prism (or 

 rather of the crystal of which it is com- 

 posed) is in the plain of primitive pola- 

 risation, the one image will be red, and 

 the other image green ; and if the two 

 are made to cross one another, the over- 

 lapping portions will be perfectly white. 

 Instead of a prism, it will be simplerto use 

 a complete rhomb of spar, having on one 

 of its faces a circular aperture, so large 

 that the two images of it seen through 

 the spar overlap each other : by substi- 

 tuting this for the prism, the right hand 

 portion of the one image will be red, and 

 the left hand portion of the other green, 

 while the intermediate or overlapping 

 pails will be perfectly white. 



If we reduce the thickness of the film 

 of sulphate of lime to 0.00046 of an 

 English inch, it will produce no colour at 

 all, having no more action upon polarised 

 licrht, than a plate of common glass. A 

 film 0.00124 of an inch thick, gives the 

 white of the first order in Newton's scale 

 of colours (see OPTICS, p. 35) ; and a 

 plate 0.01818 of an inch thick, and all 

 thicker plates, give a white composed of 

 a mixture of all the colours. Plates 

 having a thickness intermediate between 

 0.00124 and 0.01818 of an inch, produce 

 all the different orders of colours con- 

 tained in Newton's table ; and the colour 

 which any given thickness will exhibit 

 C 2 



