in the polarized Tints of certain Crystals with one axis. S39 
siderable, and indicates a nearer approach to equality between 
the extreme red and violet than in the Newtonian Scale. Struck 
by this circumstance, a passage in a letter of M. Biot, dated Oc- 
tober 21. 1819, now occurred to my recollection, in which, speak- 
ing of the conclusion I had arrived at by observations on ho- 
mogeneous light in the ordinary variety of this mineral, he says, 
“ Si vous etes bien assure de cette anomalie, je desirerai beaucoup 
que vous voulussiez bien essayer si elle se soutient a toutes les 
epaisseures, ou si elle eprouve quelque variation avec la lon- 
gueur du trajet que fait le rayon a travers la substance cristal- 
lisee. Je serais extremement curieux de savoir lequel de ces 
deux cas a lieu.'” 
To this surmise of a variation in the proportional lengths of 
the periods depending on the thickness of the plate, or the 
length of the path traversed within the crystal, all my previous 
observations had certainly enabled me to answer decidedly in the 
negative. But so singular a deviation from what I had before 
observed, led me to suppose that there might be something in 
this observation deserving a minuter examination, and I resol- 
ved to sacrifice this specimen to the inquiry. The result, as will 
be seen, by a most accidental coincidence, actually verified the 
suggestion of that acute philosopher, though in a way which 
he certainly never could have contemplated. 
The crystal is represented in Plate VII. Fig. 1. It was 
about I inch in its greatest breadth, and 0.27 inches in thickness, 
being a portion of a right prism, the plane angle (5 a c) of 
whose base was about 96^. The sides were striated longitudi- 
nally, and appeared to have been encased with a thin and 
highly polished exterior coat, of which a small portion was still 
adhering *. The structure was perfectly lamellar, the lamina 
being parallel to the base of the prism. On examining it more 
narrowly, a remarkable flaw was perceived commencing at f and 
running along parallel to the laminae. On this 1 "set the 
edge of a knife, and succeeded without difficulty, by a smart 
blow, in cleanly separating the two portions. The little irregu- 
larities in their surface being ground away, and a good polish 
communicated to them, their thicknesses were taken by the 
* The specimen, as I learn from Mr Lowry, was brought from Utbe in Swe- 
den, and was attached to a mass of oxidulous iron. 
Y 2 
