POLARISED LIGHT 355 



discs, with dried Canada balsam dissolved in benzol, as used 

 by microscopists. Such designs have clean and sharp edges, 

 and are far neater and more transparent than the former. 1 



205. Crossed and Superposed Films. The most instructive 

 mica preparations of this kind are wedges built up of 

 successively narrower strips, first designed and constructed 

 by Mr. C. J. Fox, F.R.M.S., and which I, therefore, have 

 always described as ' Fox wedges.' They are especially 

 valuable as showing the effect of crossed films. If a film 

 gives a colour due to a retardation of say one wave-length (or 

 any other), and a second film of the same thickness be crossed 

 upon it at an angle of 90, the polarised ray that was most 

 retarded in the first, is least so in the second, and it will be 

 the same as if no film were there. This is well shown by a 

 pair of similar wedges built up of eight steps each. When 

 they are crossed, along the diagonal set of squares each step 

 is crossed by the same thickness, and is therefore black. One 

 of the -wedges is mounted in a rotator, and then produces 

 very various effects according to position, for it will soon be 

 seen that a film mounted in a position to give colour (that is, 

 its planes at 45 with the plane of the polariser) has those 

 planes (or ' axes ' as often called) crossed 90 simply by 

 reversing the side placed next the other wedge. When the 

 wedges are superposed parallel, in one position the gradation 



1 The films must be very carefully laid down in superposition in the cold, 

 the balsam being only of a slight creamy consistence, and no air-bubbles being 

 left between the films. The preparation, thus laid down and centred on one 

 glass disc, must then be left for some days to dry, before fresh balsam is placed 

 on the top and the cover-glass applied. This too must be left some days, under 

 a very small weight, to dry round the edges, then baked at not much over 

 100 F. till hard enough for cleaning. These precautions are necessary to 

 prevent the films from slipping out of place. The mica axis should be scratched 

 on every piece, but these scratches practically disappear when finished, the 

 balsam being so nearly of the same refractive index. For further details of 

 mica work, the reader may refer to my paper on ' Optical Combinations of 

 Crystalline Films ' in Proc. Physical Society, 1883 (reprinted in Phil. Mag, for 

 May 1883), or to my Light, published by Macmillan & Co. 



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