24 



POLARISATION OF LIGHT. 



opens more, as shown in Fig. 29 ; and at 

 Fig. 29. 



45, it has the form shown in Fig. 30, 

 where the form and colour of the rings 

 are much more distinctly seen than in 



fig. 27. The black cross has now sepa- 

 rated into two hyperbolic curves, the 

 branches of one curve being parallel and 

 perpendicular to those of the other. 

 The form, of the rings has a general 



resemblance to that of the ellipse. A 

 certain number of them surround each 

 separate centre, which seems to be nearly 

 in one of their foci; but after the outer- 

 most of them meet at the point of in- 

 tersection of the black cross, some of 

 the rings have points of contrary flexure, 

 and they afterwards surround the two 

 centres,' as if each centre were one of 

 their foci. We shall presently be able to 

 give a more accurate description of the 

 form of these isochromatic curves, or 

 lines of equal tint. By continuing the re- 

 volution of the plate, the phenomena of 

 /#.27 will occur at 90<>, 180, and 270<>, 

 and that of fig. 3 Oat 135, 225, and 3 15; 

 but during all these changes, the form, 

 and the colours of the rings themselves, 

 suffer no change. 



If we now examine the colours of the 

 rings, it will be found that they have a 

 general resemblance to those of Newton's 

 Table, and that the zero of the different 

 orders of colours is at or near the poles 

 or centres, A B,/g\ 27, and they increase 

 outwards, as in the uniaxal system of 

 lings already described. The rings them- 

 selves increase in diameter as the plates 

 of nitre become thinner, and diminish 

 when they become thicker ; but at all 

 thicknesses the poles A and B are the 

 centres where the colours of the rings 

 originate, and, generally speaking, never 

 suffer any displacement. 



But if we reduce the plate of nitre to a 

 great degree of thinness, so that the co- 

 lour or tint produced at the intersection 

 of the arms of the black cross, or at a 

 point half way between A and B is not 

 perceptible, the whole system of rings 

 will appear to be like the uniaxal system, 

 and the black cross will not exhibit the 

 appearance above described. By thin- 

 ning the plate of nitre, we have, as it 

 were, destroyed the action of the second 

 axis at small inclinations (as will be pre- 

 sently better understood) ; but at greater 

 inclinations this axis will still modify the 

 character of the rings. 



As these poles, viz. A, B, fig. 27, are 

 points where there is no polarisation, the 

 lines passing through them may be called 

 the lines or axes of no-polarisation, a 

 long but an expressive name, and we 

 think better than the vague one of optical 

 axes*, which has been given to them. 

 The angle subtended by the poles A, B is 



* All axes in crystals are optical axes ; but all 

 axes are not axes of no-polnrisation. An expressive 

 and useful name, which contains a fact, ought never 

 to be discarded, till a better one is obtained. 



