120 Mr. Herschel on the Action of [Feb. 



tions, then, we traverse the polarised rings (PI. IV.) fig 1,* in the 

 direction of their axis of symmetry A A', passing through their 

 poles P, P', and centre O. Now if we subject to this examina- 

 tion any one of the following substances : 



Sulphate of soda? Arragonite, 



Sulphate of baryta, Sugar, 



Nitrate of potash, Hyposulphite of strontia, 



it will be seen that the tints between the poles P P' correspond 

 to lower orders of colour than would result from assuming P, ¥', 

 as the origin of the scale, and agree much better with the 

 'assumption of certain points p, p\ without the poles, as their 

 2ero, or commencement of the scale. The poles themselves too 

 instead of being absolutely black, are tinged with colour ; and 

 the tints beyond them, instead of descending in the scale from 

 the poles outwards, continue to rise till they reach their maxi- 

 mum (which is a white, more or less brilliant, or an absolute 

 black) at the points p, p' ; after which they descend again to 

 infinity. Not that in any case they coincide precisely with the 

 scale of Newton, even with this correction, but, except in extreme 

 cases, approximate to it within some moderate limit of error. 



If, on the other hand, we examine in the same manner one of 

 the following bodies : 



Tartrate of soda and potash, Sulphate of magnesia. 



Borax, Topaz, 



Mica, 



it will be found that the imaginary points, p^ p^ (which we shall 

 call the virtual poles), from which the tints must be reckoned 

 inwards and outwards, to produce the nearest possible agreement 

 "with Newton's scale, he between the poles P, P'. 



In all these crystals, as the thickness of the plate examined 

 increases, the virtual poles p p' recede from the actual ones 

 P P', at least in respect of the number of alternations of colour 

 which intervene between them : in other words, the tint deve- 

 loped in the poles, or along the apparent axes of the crystal, 

 descends in the scale of colour, as the thickness of the plate 

 increases, and vice versa. In very small thicknesses, the tints 

 approximate pretty closely to Newton's scale, or wholly coincide 

 ■with it, while in very great ones, the tint developed in the poles 

 is the composite white of the extremity of the scale. The angu- 

 lar distance, however, of the virtual poles from each other and 

 irom the axes, remains absolutely unchanged for all thicknesses; 

 and this striking fact, which I have proved by numerous and 

 satisfactory experiments, was first suggested for examination as 

 a result of theory, and would equally hold good, as will pre- 

 sently be proved, for every conceivable law of double refraction, 



♦ The whole of the figures in this paper not being referred to in the present part, 

 ihe VUU will U given entire, with the remainder of the paper, in the next viMm\)tx,m*-Ei» 



