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IX. — -On Spectra formed by the passage of Polarised Light through Double 

 Refracting Crystals. By Francis Deas, M.A., LL.B., F.K.S.E. 



(Read, 6th. June 1870.) 



It is familiarly known as one of the commonest experiments in optics that 

 when a beam of polarised light is passed through a thin film of mica or selenite, 

 and subsequently analysed either by reflection or by double refraction, two 

 colours are seen complementary to one another, and alternating with one another 

 at each 90° of a revolution of the analysing plate or prism. 



It might be expected that the coloured light thus obtained would, if thrown 

 into the form of a spectrum by means of dispersion prisms, exhibit some 

 peculiarities, and such is the case as will be seen from the following experi- 

 ments : — 



To make the experiments intelligible, it may be well in the first place to say 

 a few words about the instrument employed, and the method of using it. 



Any spectrum microscope ought to answer the purpose provided that in 

 addition to the spectroscopic arrangement a pair of Nicol's prisms can be 

 attached, one below the stage and the other over the eye piece. Both should 

 be capable of being rotated, and it tends much to facility of working as well as 

 to exactness of result that both the polarising and the analysing prism should 

 carry graduated heads, so that their axes may readily be turned to any re- 

 quired degree of inclination to one another. 



The instrument I employed was a large Smith and Beck. The spectro- 

 scopic arrangement consists of an adjustable slit attached to the under part of 

 the substage below the achromatic condenser, and a set of direct vision prisms 

 which are inserted in the body of the instrument immediately above the object 

 glass. 



By proper focusing, an image of the slit is thus formed by the achromatic 

 condenser in the focus of the object glass, and a fine spectrum obtained filling 

 the whole field. 



This arrangement, it will be seen, differs considerably from the spectrum 

 microscope in common use in which the dispersion prisms are placed close to 

 the observer's eye, the slit being in the focus of the eye lens. The former 

 arrangement has this manifest advantage, that owing to the distance of the 

 prisms from the eye, the spectrum fills the whole field ; also, that the apparent 

 breadth of the spectrum can be varied at pleasure by a change of the magnify - 



VOL. XXVI. PART I. 3A 



