95 
crystallized bodies on homogeneous light. 
what results I had obtained respecting the deviation of the tints from Newton s 
scale. The following general points will enable you to judge of the progress which I 
had made in the enquiry. 
« i. In almost all crystals with two axes there is a deviation from the tints ot 
Newton’s scale. 
“ 2. This deviation is greater in some crystals than in others, being a maximum in 
acetate of lead and tartrate of potash and soda. 
“ 3. That all these crystals may be divided into two classes, viz. those which 
have the red ends of the rings inwards and the blue ends outwards, and those which 
have the red ends outwards and the blue ends inwards. 
“ 4. That in all crystals with two axes, the doubly refracting force of one axis in 
general acts differently upon the differently coloured rays from the doubly refracting 
force of the second axis. 
“ 5. That as the polarising force is always proportional to the force of double 
refraction, the polarising force of one axis will act differently on the differently 
coloured rays from the polarising force of the other axis. 
€€ A . . . 
“ 7. The consequence of this is, that there will be different resultant axes, or dif- 
ferent points of compensation for the differently coloured rays. 
« 8. All these effects may be calculated with the utmost accuracy, if the ratio of 
the dispersive powers of the two extraordinary refractive forces is given, or vice versd, 
the dispersive powers may be obtained from the angles of the resultant axes for the 
red and violet rays of the spectrum. 
9. I have found crystals in which these phenomena art decidedly connected 
with the rotatory phenomena ; and from this highly important fact I am led to 
conclude, that both have the same origin, and that all the rotatory phenomena are, 
as I have stated in my paper, the result of the uncompensated tints of two axes, 
equal for the mean ray, but unequal for all the rest. {Here follows an illustration 
by a diaphragm.) 
“ 10. The division into two classes in $ iii. as founded merely on observation, is 
converted into another division into two classes, viz. 1. That in which the doubly 
refracting force of the principal axis acts more powerfully on the blue rays than the 
other axis does j and 2. That in which it acts less powerfully. The first class com- 
prehends those crystals in which the blue ends are inwards, and the second those in 
which the red ends are inwards, or nearer the principal axis.” 
In a subsequent letter (Oct. 4), he adds, 
“ The virtual poles, which you mention, I discovered in the year 1815, and I have 
two accounts of them in my Journal, the one signed on the 24th January, 1816, and 
