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III. — On Spectra formed hy the passage of Polarized Light 

 through Dovhle-refrading Crystals seen with the Microscope. 



By Fkancis Deas, M.A., LL.B., F.R.S.E. 



Plate XCV. 



It is familiarly known as one of the commonest experiments in 

 optics that when a beam of polarized light is passed through a thin 

 film of mica or selenite, and subsequently analyzed either by re- 

 flexion 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 analyzing 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 disper- 

 sion prisms, exhibit some peculiarities, and such is the case, as will 

 be seen from the following experiments : — 



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 polarizing and the analyzing prism should carry graduated 

 heads, so that their axes may readily be turned to any required 

 degree of inchnation to one another. 



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

 spectroscopic arrangement consists of an adjustable slit attached to 

 the under-part of the sub-stage 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 focussing, 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. 



EXPLANATION OF PLATE XCV. 



Fig. 1. — Illustrating the dividing and re-union of the bands, as described. 



„ 2. — Spectra formed by ordinary and extraordinary ray, partly overlapjDmg. 

 The blue of the upper spectrum seen through the black band belong- 

 ing to the lower spectrum. The yellow of the under'spectrum seen 

 tlirough the black band of the upper. 



„ 3. — Eings and brushes of crystal of sugar-candy. 



„ 4. — Showing displacement of the rings and absence of brushes of same crystal 

 when light circularly polarized before and after its passage through 

 the crystal. 



„ 5. — Lemniscates and brushes of crystal of nitrate of potash. 



VOL. VI. L 



