J. II. HAEFORD POLARISATION OF LIGHT. 159 



light remains unchanged, i.e. white. If the difference amounts to 

 more or less than an entire wave, the two reflected waves are in a 

 condition of interference, part of the vibrations is suppressed or 

 neutralised by the mutual opposition, and a residue only is ob- 

 served, and this residue is coloured. The tint of colour will be 

 according to the amount of interference, or, pari passu, the thick- 

 ness of the film. In the case of the double refraction it will be 

 remembered that the refracted beams pass through strata of 

 unequal densities, and therefore unequal refi'active power. One, 

 therefore — i.e. the ordinary ray — suffers more obstruction than 

 the other, or, to use the recognized word, is "retarded," and this 

 retardation serves the same office as reflexion and refraction in the 

 former instance, when by means of a proper contrivance two beams 

 in the same plane of polarisation, but in different phases as to 

 vibration, are made concurrent. Keference to Fig. 14 will help 

 to make this apparent. Let a represent a beam of polarised light. 

 This passing through a thin slice of selenite h cut parallel to the 

 axis of the crystal is separated into two rays c and c' . This 

 separation, however, is so small as to be invisible. The light may 

 simply be said to be dislocated, or the separation started. This 

 light is then analysed by passing through a crystal of Iceland 

 spar d which has the power of completely separating the rays, and 

 each ray c and c' is divided into two, making /o<<r in all — e e' derived 

 from one ray, say the ordinary ray, and //' from the other, the 

 extraordinary ray. In each case the separation is accompanied by 

 polarisation in opposite planes. This separation into four is in- 

 visible. The visible result is that two rays g and g' are seen, each 

 being compounded of two rays respectively in the same plane, but 

 in a state of interference. For note that g is formed by concurrence 

 of e and/, and g' by concurrence of e' and /'. Now e and / come 

 from different sources — e from the ordinary ray, and / from the 

 extraordinary ray, and are consequently in different phases of 

 vibration. The same may be stated in reference to e' and /'. 

 The colours in the result are complementary, i.e. by their union 

 white light is made complete, one colour being the complement 

 to the other in this respect. 



This paper being intended simply as an abstract of a lecture on 

 the subject, in the course of which specimens, apparatus, and 

 diagrams were freely used, it is obvious that to render the paper as 

 complete and effective as the lecture it would expand into a treatise 

 and prove unnecessarily voluminous. One other topic only, con- 

 cerned with the subject, will be alluded to.. The consideration 

 has hitherto been devoted to what is termed plane polarisation, 

 meaning that the polarisation that takes place has reference to two 

 planes at right angles to each other. The term rectangular polar- 

 isation is sometimes more correctly applied. There are, however, 

 phenomena of polarisation that belong not to two rectangular 

 planes only, but are exhibited at all planes in succession around the 

 central point. The phenomena of colour in this case, instead of 

 taking place in reference to two opposite planes alternately, are 



