2i 6 Dr, Brewster on the Affections of Light 
fringes 3° 12' in breadth. Now 
135 : it)8 = 3° 12' : 4® nearly, 
so that the diameters of the rings are inversely as tlie thick- 
nesses of the plates, as in the case of toj)az. The light was 
incident on the sulphate of potash at an angle of which 
gives for the oblique thickness of the plate 7^0 of an 
inch thick. Now 
i 7 9 
1000 
1° aT 
^ °9 . pO o' 
I O O O • " > 
the size of the ring that would have been produced by a 
plate of topaz y ~ q^ q of an inch thick, so that the thicknesses 
of sulphate of potash and topaz that produce equal rings are 
as 1.85 to 1, which is not very far from the ratio of {m — 1)^ to- 
(m' — i)\ If we take 1.509 and m' = 1.6^6 this ratio 
will be as 1.95 to 1. 
10. Tartrate of potash and soda. The neutral axes of this salt 
are parallel and perpendicular to the axis of the prism, and it 
possesses an oblique depolarising axis along which the coloured 
rings are visible. The thicknesses of this substance and of 
topaz, at which equal rings are produced, are as 31 : 1^, 
which is almost exactly the ratio of {m — i)^to {ni' — 1)^. 
The value of 7n' in the tartrate being 1.515. 
11. Nitrate of potash. This salt, which is remarkable for its 
optical properties,^ exhibits along the axis of the hexaedral 
prism a series of beautiful miniature rings, twelve of which are 
distinctly visible. In a plate of the nitrate of potash of 
an inch thick, the fourth ring subtended an angle of 5° 45', 
whereas, in a plate of topaz 7^^^ of an inch thick, it sub- 
tended an angle of 8° 25'. But 
* I have endeavoured to give a full account of these in the Transactions of the 
Royal Society of Edinburgh, Vol. VII. Part II. 
