108 Prof. Stokes on the Occurrence of Abnormal Figures in 



peculiar, that the chances may be regarded as infinity to one that 

 no such changes of form will be produced to any material ex- 

 tent. 



But when photography is applied to the representation of 

 phamomena of interference, such as the rings of crystals seen by 

 means of polarized light, the case is in some respects materially 

 different. To take a particular instance, let us suppose the 

 rings of calcareous spar to be viewed by white light, the planes 

 of polarization of the polarizer and analyzer being crossed, so as 

 to give the black cross ; and consider the alternations which take 

 place in going outwards from the centre of the field, suppose in 

 a direction inclined at angles of 45° to the arms of the cross. 

 At first there are evident alternations of intensity; but very 

 soon the eye, which under such circumstances is but a bad judge 

 of differences of intensity even when the lights to be compared 

 have the same colour, can no longer perceive the differences of 

 illumination, but judges entirely by the difference of tint. The 

 same takes place with nitre, sugar, and other colourless biaxal 

 crystals. Except in the immediate neighbourhood of the optic 

 axis or axes, the rings, which owe their existence and their forms 

 in the first instance to the laws of double refraction and of the 

 interference of polarized light, are in other respects created and 

 their forms determined by the condition of maximum contrast of 

 tint. 



Now consider what takes place when an image of such a 

 system is thrown on a sensitive plate, prepared suppose by 

 means of bromide of silver. The rays of any one refrangibility 

 would together form a regular system of rings, which, if these 

 rays were alone present, and if the refrangibility were comprised 

 within the limits between which the substance is acted on, would 

 impress on the plate a system of rings exactly like those seen by 

 means of the same homogeneous rays, provided they belong to 

 the visible spectrum. The same would take place for rays of each 

 refrangibility in particular, and the several elementary systems 

 of rings thus formed are actually superposed when heterogeneous 

 light is used. When the photograph is finished, it exhibits 

 certain alternations of light and shade corresponding to alterna- 

 tions in the total photographic intensity of the rays which had 

 acted on the plate, without any distinction being preserved be- 

 tween the action of rays of one refrangibility and that of rays of 

 another ; whereas, when the rings are viewed directly, the eye 

 catches the differences of tint without noticing the difference of 

 intensity, except in the neighbourhood of the optic axis or axes. 

 Of course I am now speaking only of the alternations perceived 

 in following a line drawn across the rings, not of the dark 

 brushes, or of the variation of intensity perceived in passing 



