384 



Sir C. Wheatstone on the 



[Mar. 23, 



crystals, all agree therefore in this respect : — that if the plane of polarization 

 of the quickest ray is to the left of the plane of reflection, the successive 

 polarization is right-handed when the analyzer moves from left to right ; 

 ancf if it is to the right of the plane of reflection, other circumstances 

 remaining the same, the successive polarization is left-handed. 



It must be taken into consideration that the principal section of the 

 film is inverted in the reflected image ; so that if the plane of polarization of 

 the quickest ray in the film is to the left of the plane of reflection, it is to 

 the right of that plane in the reflected image. 



IV. 



, It may not be uninteresting to state a few obvious consequences of this 

 successive polarization in doubly refracting laminae, right-handed and 

 left-handed according to the position of the plane of polarization of the 

 quickest ray. They are very striking as experimental results, and will 

 serve to impress the facts more vividly on the memory. 



1. A film of uniform thickness being placed on the diaphragm with its 

 principal section 45° on either side the plane of reflection, when the 

 analyzer is at 0° or 90° the colour of the film remains unchanged, whether 

 the film be turned in its own plane 90°, or be turned over so that the back 

 shall become the front surface; but if the analyzer be fixed at 45°, 135°, 

 225°, or 315°, complementary colours will appear when the film is in- 

 verted from back to front, or rotated in its own plane either way 90°. 



2. If a uniform film be cut across and the divided portions be again 

 placed together, after inverting one of them, a compound film (fig. 4) is 

 formed, which, when placed on the diaphragm, exhibits simultaneously both 

 right-handed and left-handed successive polarization. "When the analyzer 

 is at 0° or 90° the colour of the entire film is uniform ; as it is turned 

 round the tints of one portion ascend, while those of the other descend ; 

 and when the analyzer is at 45° orrc90° + 45°, they exhibit complementary 

 colours. 



3. A film increasing in thickness from one edge to the other is well 

 suited to exhibit at one glance the phenomena due to films of various 

 thicknesses. It is well known that such a film placed between a polarizer 

 and an analyzer will show, when the two planes are parallel or perpen* 

 dicular to each other, and the principal section of the film is intermediate 

 to these two planes, a series of parallel coloured bands, the order of the 

 colours in each band from the thick towards the thin edge being that of 

 their refrangibilities, or R, O, Y, G, B, P, V. The bands seen when the 

 planes are perpendicular are intermediate in position to those seen when 

 the planes are parallel ; on turning round the analyzer these two systems 

 of bands alternately appear at each quadrant, while in the intermediate 

 positions they entirely disappear. , : 



Now let us attend to the appearances of these bands when the wedge- 

 form film is placed on the diaphragm of the instrument, fig. 1. As the 



