118 Mr. Herschel on the Action of [Feb. 



first orders " of its rings ; and while he remarks that such devia- 

 tions are common enough, and indeed universal in crystals " in 

 which the rings are formed by the joint action of two axes," 

 seems to think this analogy close enough to authorize the sub- 

 stitution of two rectangular axes of a negative character for the 

 single positive axis actually observed, according to his own 

 peculiar and ingenious views on this subject. I lost no time in 

 endeavouring to procure a specimen of this mineral, and by the 

 kindness of my friend. Sir Samuel Young (to whom I owe more 

 than one obligation of this nature) was favoured with one suffi- 

 ciently transparent for optical examination. From my observa- 

 tions on this body, I think I shall be able to demonstrate satisfac- 

 torily that the phenomena of the apophyUite depend on a 

 principle distinct from that which produces the chief part of the 

 deviation of tints in most crystals with two axes. 



The course I propose to pursue is, first, to describe the phe- 

 nomena themselves. I shall then show how these phenomena, 

 complicated as they are in appearance, are all reducible to one 

 very simple and general fact; viz. that the axes of double 

 refraction differ in their position in the same crystal for the dif- 

 ferently coloured rays of the spectrum, being dispersed in one 

 {jllane over an angle more or less considerable, according to the 

 nature of the substance. In many bodies, the magnitude of this 

 dispersion of the axes is comparatively trifling, while in some, 

 not otherwise remarkable for a high ordinary or extraordinary 

 dispersive power, it is enormous, and must render all computa- 

 tion of the tints in which it is not taken into consideration, com- 

 pletely erroneous ; and indeed obliterating almost every trace of 

 the Newtonian scale of colour. We have here then a new ele- 

 ment, which, for the future, must enter into all formulas of double 

 refraction pretending to rigour, and at the same time are pre- 

 sented with another very striking instance of the inherent dis- 

 tinction between the differently coloured molecules of light, 

 which, since the time of Newton, every new step in optical 

 science has tended to place in a stronger point of view. At the 

 same time, by the easy and complete explanation this principle 

 affords of all the more perplexing anomalies in the tints, the 

 theory of alternate polarisation to which they were hitherto so 

 palpable and formidable an objection, stands relieved from every 

 difnculty, and may now be received as fully adequate to the 

 representation of all the phenomena of the polarised rings, and 

 entitled to rank with the fits of easy transmission and reflection, 

 as a general and simple physical law. In fact, if we investigate 

 by this theory a general analytical expression of the tint deve- 

 loped for any position and thickness of the plate, taking this 

 ^ment into consideration, it will be found to include all the 

 phenomena, as far as they can be computed ; while the law of 

 dispersion remains unknown. But we may go yet further. The 



