THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



— -&. — • 



[THIRD SERIES.] 



MARCH 1848. 



XXIV. On the Distinctness of Vision produced in certain cases 

 hy the use of the Polarizing Apparatus in Microscopes. Bij 

 Sir David Brewster, K.H., D.C.L., F.R.S.and F.P.R.S. 

 JEdin.* , 



HAVING lately had occasion to examine some very mi- 

 nute crystals, and also some animal and vegetable fibres 

 that possessed the doubly-refracting structure, I was surprised 

 to find that, by the use of the polarizing apparatus, I could 

 eliminate two kinds of indistinctness which affect the vision of 

 microscopic objects. The interposition of a Nicol's prism, or 

 of an analysing rhomb of calcareous spar, however skilfully 

 formed, between the eye and the object, has always been con- 

 sidered as deteriorating the microscope, and the observer is 

 justified in removing it in ordinary cases when he wishes to 

 obtain the most perfect definition which his instrument can 

 give. Wlien the object, however, has a doubly-refracting 

 structure of the slightest kind, so as to act upon polarized 

 light, the polarizing apparatus is of vast service in developing 

 its form and structure, not merely its doubly refracting struc- 

 ture, but that form and structure which it exhibits in common 

 light. 



In order to illustrate this use of the polarizing apparatus, 

 let us take the seed of the Collomea grandiflora, which, when 

 steeped in water, throws out hundreds of spiral fibres like 

 corkscrews, with the spires sometimes elongated by pressure 

 into waving lines, and sometimes compressed almost into con- 

 tact f. If we now place these s})irals between two plates of 

 glass and in castor-oil which has nearly the same refractive 

 power as the fibres, we shall obtain a tolerably distinct view 



• Communicated by the Autlior. 



t This elongation and compression of the spirals was isroduced by 

 prcssini; the spirals out of the seed when steeped in castor oil. 

 Phil. Ma<r. S. 3. Vol. 32. No, 2 1 1. March 1 848. M 



