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of Edinburgh , Session 1869 - 70 . 
isation. The positions and relative movements of the bands de- 
pend partly on the thickness of the films, partly on the inclination 
of their axes to one another, and to the planes of polarisation as 
detailed in the paper. 
Curious varieties of the movements are obtained by circularly 
polarising the light before or after its passage through the film. 
Very beautiful results were further obtained by substituting a 
double image prism as the analyser. When the spectra thus ob- 
tained are superposed, the bands are no longer black, but coloured, 
each band in the one spectrum being of the colour of that part of 
the other spectrum on which it is superposed, while the adjacent 
colours are those arising from the blending of the two spectra. 
To obtain these effects in perfection, however, certain adjust- 
ments of the apparatus must be attended to, which will be found 
described in the paper. 
The second part of the paper relates to the effects obtained 
when a section of a doubly refracting crystal, cut perpendicular to 
its axis, so as to give the well-known systems of coloured rings, is 
substituted for the mica or selenite in the former experiments. 
The crystal must in this case be placed, not upon the stage, but 
immediately over the eye lens of the instrument, and between it 
and the analyser. The entire length of the spectrum is now seen 
intersected by a system of black arcs, accompanied by two or more 
brushes, which are black or coloured according to the position of 
the analyser. 
Interesting effects are produced upon the rings by interposing 
films of mica of different thicknesses, so as to polarise the light 
either circularly or elliptically ; the mode in which the black and 
coloured rings alternate and change places during the revolution of 
the analyser depending on the thickness of the film used. 
The effect of the rings, when viewed through a double image 
prism, is strikingly beautiful. Exquisite patterns resembling tessa- 
lated pavement, chain armour, &c., may thus, with a little inge- 
nuity in the mode of arrangement, be produced by the interlacing 
-systems of rings. 
VOL. VII. 
