Dr. Brewster on the laws of polarisation, &c. 205 
have been able to ascertain whether they have one or more 
axes ; and by examining and measuring the tints which they 
exhibit at various angular distances from the axes whence 
the forces emanate, I have been led to a general principle 
which embraces all the phenomena, and extends to the most 
complex, as well as to the most simple developement of the 
polarising forces. This general principle is in no respect an 
empirical expression of the facts which it represents, nor is it 
supported by any empirical data. Founded on the principles 
of mechanics, it is a law rigorously physical, and enables us 
to calculate all the tints, &c. of the coloured rings, and all 
the phenomena of double refraction, with as much accuracy 
as we can compute the motions and positions of the heavenly 
bodies. 
In the course of researches embracing the examination of 
such a great number of crystals, it was natural to expect that 
many new facts would present themselves that were but 
slightly connected with the general object of investigation. 
The phenomena of this kind, which I have had the good 
fortune to observe, comprehend several new properties of 
light, and of crystallized bodies, which 1 shall lay before the 
Society in a series of separate communications. 
Section I. On the crystals which produce double refraction. 
The property of double refraction was observed by the 
Abbe Hauy in twenty crystallized substances.* Malus has 
given a list of nineteen doubly refracting crystals, embracing 
Traiie de Mineralogie, tom, i. p. 271, 272, Paris, 1801. 
