172 
Proceedings of the Royal Society 
article “ Mechanics Applied,” of the “ Encyclopsedia Britannica,” 
eighth edition ; and the next again in 1858, in a work of mine 
entitled “ A Manual of Applied Mechanics.” 
Mr Clerk Maxwell made a material improvement in the mode of 
applying the method to braced frames, which he published in the 
u Philosophical Magazine ” for 1866, and described to the Dundee 
meeting of the British Association. — I am, Sir, your most obedient, 
servant, W. J. Macquorn Rankine. 
Glasgow, 2d June 1870. 
The following Communications were read : — 
1. On Spectra formed by Doubly Refracting Crystals in 
Polarised Light. By Francis Deas, Esq., LL.B., F.R.S.E. 
The instrument used in the experiments forming the subject of 
this paper was a spectrum microscope, to which a polarising appara- 
tus is attached, consisting of two NicoFs prisms, each of which is 
capable of being turned through any required number of degrees. 
The first part of the paper relates to the spectra obtained when 
one or more thin films of mica or selenite are interposed between 
the polariser and the dispersion prisms, the light being subse- 
quently analysed. 
The method employed was, having first determined the axes of 
the films, to place them on the stage of the instrument which is 
rotatory, and to adjust them at various angles to the plane of 
polarisation. 
The general appearance presented, may be described as being a 
more or less continuous spectrum, interrupted by one or more well 
defined black bands, not unlike the ordinary absorption bands pro- 
duced by many chemical substances. 
The bands have in many cases a curious movement along the 
length of the spectrum as the analyser is turned. Sometimes a 
band may be observed to split into two halves, which move in op- 
posite directions, and unite with other bands which advance to 
meet them. 
In all cases a set of complementary bands is obtained when the 
plane of analysation has been turned through 90° to that of polar- 
