in the polarized Tints of certain Crystals with one axis. 343 
immediately in the rich tints of indigo, purple and crimson, 
which occur in the first order. The yellow rays, too, afford us 
a numerical verification of the number assigned to them. The 
maonima and minima of these coincide, as Newton has observed, 
with the most luminous, and obscurest parts of the rings, which 
is a necessary consequence of their great illuminating power. 
Now these occur at the incidences 23° 35', 33° 2', and 39* 41' 
respectively; and if we compute the angles of refraction (^) cor- 
responding to these, and take n successively 1, |, the formu^ 
la already employed gives 
by the first maximum - - I ~ 20269 
by the first minimum --/=?: 29822 
by the second maximum - I — 29370 
I =1 29487 Mean. 
'which differs from the result in the third Table by less than 
of its value. 
The absolute polarizing powers of the two portions into which 
the crystal was divided, differed no less remarkably than the 
characters of their tints. In the thicker plate, by a mean of 20 
careful observations made by the interposition of a certain stand- 
ard red glass, on the ring of the third order at its minimum^ (in 
which the evanescence of the extraordinary pencil was complete,) 
I found 37° 3' for its apparent semi-diameter in air ; and hence 
we find 6 = 23° 7', = 3, t — 165900, which substituted give 
z= 9269 ; = 107.886, 
V 
and, (as is sufficiently evident from the scale of tints in this por^ 
tion,) the value of Z is nearly the same for all the other colours. 
Now it is well worthy of observation, that this value coincides 
almost precisely with the number similarly determined for the 
variety examined in my paper above alluded to, which I have ’ 
there found to 9281. The difference is httle more than 
of the whole, and so exact an agreement could hardly have 
been expected even in plates detached from the same specimen. 
This circumstance, together with the identity in the scale of 
tints exhibited by the two substances, establishes not only their 
exact similarity as individuals, but, what is of much more im-, 
portance in this case, the definite nature of the variety itself ; 
