Feb. 26, 1874] 



NATURE 



525 



" It must be taken into consideration that the principal 



section of the film is inverted in the i-eflected image ; so 

 that if the plane of polarisation of the quickest ray in the 

 film is to the left of the plane of reflexion, it is to the right 

 of that plane in the reflected image. 



"It may not be uninteresting to state a few obvious 

 consequences of this successive polarisation in doubly re- 

 fracting lamina:, right-handed and left-handed according 

 to the position of the plane of polarisation of the quickest 

 ray. They are very striking as e.xperimental results, and 

 will serve to impress the facts more vividly on the 

 memory. 



" I. A film of uniform thickness being placed on the 

 diaphragm with its principal section 45^ on either side 

 the plane of reflexion, when the analyser 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 

 analyser be fixed at 45°, 135°, 225°, or 315°, comple- 

 mentary colours will appear when the film is inverted 

 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 is formed, which, when 

 placed on the diaphragm, exhibits simultaneously both 

 right-handed and left-handed successive polarisation. 

 When the analyser 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 analyser is at 45° or ji')o° -\- 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 pheno- 

 mena due to films of various thicknesses. It is well 

 known that such a film placed between a polariser and an 

 analyser will show, when the two planes are parallel or 

 perpendicular 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 analyser these two systems of bands 

 alternately appear at each quadrant, while in the inter- 

 mediate 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. 15. As the analyser is moved round, 

 the bands advance toward or recede from the thin edge 

 of the wedge without any changes occurring in the colours 

 or intensity of the light, the same tint occupying the same 

 place at every half revolution of the analyser. If the 

 bands advance toward the thin edge of the wedge, the 

 successive polarisation of each point is left-handed ; and 

 if they recede from it the succession of colours is right- 

 handed : every circumstance, therefore, that with respect 

 to a uniform film changes right-handed into left-handed 

 successive polarisation, in a wedge of the same substance 

 transforms receding into advancing bands, Tscad^viccvcrsa. 

 These phenomena are also bcautitully shown by concave 

 or convex films of selenitc or rock-crystal, which exhibit 

 concentric rings contracting or exp.anding in accordance 

 with the conditions previously explained. 



" 4. Few experiments in physical optics are so beau- 

 tiful and striking as the elegant pictures formed by 

 cementing lamina; of selenite of different thicknesses 

 (varying from oo\,o to 5'^ of an inch) between two 

 plates of glass. Invisible under ordinary circumstances, 

 they exhibit, when examined in the usual polarising- 

 apparatus, the most brilliant colours, which are comple- 

 mentary to each other in the two rectangular positions of 

 the analyser. Regarded in the instrument, Fig. 13, the 



appearances are still more beautiful ; for, instead of a 

 single transition, each colour in the picture is succes- 

 sively replaced by every other colour. In preparing such 

 pictures it is necessary to pay attention to the direction of 

 the principal section of each lamina when different pieces 

 of the same thickness are to be combined together to 

 form a surface having the same uniform tint ; otherwise 

 in the intermediate transitions the colours v/ill be irregu- 

 larly disposed. 



" 5. A plate of rock-crystal cut perpendicular to the 

 axis loses its successive polarisation, and behaves exactly 

 as an ordinary crystallised film through which rectiUnear 

 polarised light is transmitted. . 



" By means of the phenomena of successive polarisa- 

 tion it is easy to determine which is the thicker of two 

 films of the same crystalline substance. Place one of the 

 films on the diaphragm E of the instrument (Fig. 15) in 

 the position to show, say, right-handed polarisation, then 

 cross it with the other film ; if the former be the thicker, 

 the successive polarisation will be still right-handed ; if 

 both be equal, there will be no polarisation ; and if the 

 cross film be the thicker, the successive polarisation will 

 be left-handed. In this manner a series of films may be 

 readily arranged in their proper order in the scale of 

 tints. 



" In the experiments I have previously described the 

 planes of reflexion of the polarising minor and of the 

 silver plate were coincident ; some of the results obtained 

 when the azimuth of the plane of reflexion of the silver 

 plate is changed are interesting. 



" I will confine my attention here to what takes place 

 when the plane of reflexion of the silver plate is 45° from 

 that of the polarising reflector. 



" When the principal sections of the film are parallel 

 and perpendicular to the plane of reflexion of the polar- 

 ising mirror, as the whole of the polarised light passes 

 through one of the sections, no interference can take 

 place, and no colour will be seen, whatever be the position 

 of the analyser. 



" When the principal sections of the film are parallel 

 and perpendicular to the plane of reflexion on the silver 

 plate, they are 45° from the plane of reflexion of tVe 

 polarising mirror. 



" The polarised ray is then resolved into two compo- 

 nents polarised at right angles to each other ; or;e com- 

 ponent is polarised in the plane of reflexion of the silver 

 plate, the other perpendicular thereto ; and one is 

 retarded upon the other by a quarter of an undulation. 



"When the analyser is at o" or 90" no colours are seen 

 because there is no interference ; but when it is placed at 

 45^ or 135°, interference takes place, and the same colour 

 is seen as if light circularly polarised had been passed 

 through the film. The bisected and inverted film shows 

 simultaneously the two complementary colours. 



'• But when the film is placed with one of its principal 

 sections 22j° from the plane of reflexion of the polarising 

 mirror, on turning round the analyser the appearances 

 of successive polarisation are rep',oduced exactly as when 

 the planes of reflexion of the silver plate and of the 

 polarising mirror coincide. In this case the components 

 of the light oppositely polarised in the two sections are 

 unequal, being as cos 22;}' to sin 22!^° ; these components 

 respectively fall 22i' from the plane of reflexion of the 

 silver plate and from the perpendicular plane, and are 

 each resolved in the same proportion in these two planes. 

 The weak component of the first, and the strong compo- 

 nent of the second, are resolved into the normal plane, 

 while the strong component of the first and the weak 

 component of the second are resolved into the perpen- 

 dicular plane. 



"The apparatus (Fig. 15) affords also the means of 

 obtaining large surfaces of uncoloured or coloured light 

 in every state of polarisation— rectilinear, elliptical, or 

 circular. 



