50 Mr. J. F. W. Herschel on the action of 
of double refraction differ in their position in the same crystal 
for the differently coloured rays of the spectrum, being dis- 
persed in one plane 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 compara- 
tively trifling, while in some, not otherwise remarkable for 
a high ordinary or extraordinary dispersive power, it is 
enormous, and must render all computation of the tints in 
which it is not taken into consideration, completely erroneous ; 
and indeed obliterating almost every trace of the Newtonian 
scale of colour. We have here, then, a new element, which 
for the future must enter into all formulae of double refraction 
pretending to rigour, and at the same time are presented with 
another very striking instance of the inherent distinction be- 
tween 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 pal- 
pable and formidable an objection, stands relieved from every 
difficulty, 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 developed for any position and thickness of the plate, 
taking this element into consideration, it will be found to in- 
clude all the phenomena, as far as they can be computed. 
