1821.] Crystallized Bodies on Homogeneous Light, 119 



nature of the formula furnishes an equation by which the actual 

 quantity of the separation of the extreme red and violet axes 

 may be deduced from observations of the tints of a very simple 

 andf accurate nature, being perfectly analogous in principle to 

 the '*' method of coincidences," which has of late been appHed 

 with such success to the most delicate investigations in every 

 department of physical science. The comparison of the results 

 afforded by that equation with those deduced by direct observa- 

 tion on homogeneous hght ; while it leaves nothing to desire in. 

 point of accuracy, leads to another important result, viz. that the 

 proportionality of the minimum lengths of the periods performed 

 by differently coloured molecules, in a doubly refracting crystal 

 to the lengths of their fits of easy reflection and transmission, 

 supported as it is by an induction of no ordinary extent and 

 accuracy, is yet not universal, admitting a deviation to a very- 

 large amount. Hence must of course arise a kind of secondary 

 deviation in the scale of tints. In crystals with two axes, how- 

 ever, this is masked by the much more powerful effect of the 

 separation of the coloured axes ; yet even there, is not altoge- 

 ther insensible in an extreme case. In the apophyllite, however, 

 the agency of this secondary cause is placed in the fullest evi- 

 dence. The appUcation of our general formula to the anomalous 

 tints of that body, while it proves incontestably the exact coinci- 

 dence of the axes for all the coloured rays, points out at the 

 same time a pecuharity in its action on the more refrangible 

 extremity of the spectrum, of a nature so singular, so entirely 

 without example in all the multitude of natural and artificial 

 bodies hitherto examined, as to render me extremely desirous of 

 prosecuting the research, with the aid of more perfect specimens 

 and improved methods of observation. 



Having arrived at the general result of a dispersion of the axes 

 by the sole consideration of the gradation of tints in plates of 

 various thicknesses, it becomes interesting to verify it by direct 

 and independent observation. This I have accordingly done ; 

 and the fortunate discovery of a substance in which it is of 

 enormous magnitude, puts it in our power to render the fact 

 sensible to the eye of the most unpractised observer, by an 

 exceedingly simple experiment, to be described in its place. 



II. Of the general Phenomena of Crystals which develope Tints 

 deviating from Newton's Scale j by Exposure to polarised Light, 



In describing the phenomena, I shall at present confine myself 

 to the tints developed along the principal section of the crystal, 

 which is supposed placed in an azimuth 45° with the plane of 

 primitive polarisation. The observations of the tints in this 

 position are most easily made, and least liable to error, and we 

 shall see presently that it would be superfluous as well as embar- 

 rassing to examine other situations, the law of the phenomena 

 being completely deducible from this. In this series of observa^ 



