526 The Rev. Professor O'Brien on the 



is for >j, which explains the peculiar nature of the absorption 

 of tourmaline. 



24. To explain the rotation produced by certain fluids in 

 the plane of polarization of a ray transmitted through them, it 

 is necessary to show that the velocity of propagation of circu- 

 larly polarized light in the fluid is different for right-handed 

 and left-handed rotation. How the fluid can have this effect 

 on light, no matter in what direction it is transmitted through 

 it, is a very great difficulty, and has not been explained so far 

 as I know. I think however that this difficulty may be got 

 over by supposing that the particles of the fluid are compound, 

 each being a group of other particles arranged in such a man- 

 ner that any section of one of these groups Fig. 1. 

 is something like what is represented in 

 the annexed figure (fig. 1). These groups •© ^ 

 are supposed to be perfectly similar in all • * • ^ « * 

 respects, and symmetrically posited ; so • • • % ^^ 

 that, if any line be drawn through thecen- • ^*«*«e« 

 tre of one of the groups, not only that ^ • 

 group, but the whole system of groups are • • 

 placed symmetrically about that line. A 

 little consideration will show that the particles composing any 

 group may be so arranged that, if we represent those which 

 lie in any particular section of that group in a figure as above, 

 they will appear to be as it were twisted from right to left, and 

 form intersecting lines of particles, each in the shape 2. In 

 the same manner the particles might be arranged in such a 

 manner that those in any section^would appear to be twisted 

 from left to right, as in fig. 2, and form intersecting lines of 

 particles in the shape 3 • 



Let A (fig. 3) represent any one of the compound particles 

 of the transparent substance, such suppose as is represented 

 in fig. 2; and let A B C be the circle which any point of the 



Fig. 3. 

 Fig. 2. /-^^ 



• -•' 



• ^ o ® 



cethereal fluid describes (the vibrations being circular). Then, 



