162 Sir D. Brewster on the Distinctness of Vision 



of them without the polarizing apparatus ; but we shall find 

 it very diflficiilt, if not utterly impracticable, to trace the fibres 

 through their circumvolutions and obtain a satisfactory defi- 

 nition of them. This difficulty arises from the diffraction of 

 light; the rays which pass the edges of the fibres interfering 

 not only with one another, but with the rays which pass 

 through the fibres, and producing great indistinctness of vision. 



If we nowexpose the spiralfibres to polarized light, and apply 

 the analysing rhomb, we shall see them beautifully delineated 

 on a dark ground in the light which they depolarize and trans- 

 mit. Spirals which were imperfectly seen before will be seen 

 distinctly now ; and the minute points or duplications of the 

 fibre which mark each spire, when the whole is drawn out 

 nearly into a straight line, will be beautifully defined, though 

 they were entirely invisible in common light. By turning 

 round the analysing rhomb and making the field of view alter- 

 nately dark and luminous, we shall be able to compare the two 

 modes of vision which I have described. • 



The cause of the superior definition thus obtained it is not 

 difficult to discover. I have elsewhere shown that the imper- 

 fect definition obtained by microscopes arises from the diffi'ac- 

 tion of light; and I have pointed out the method of illumina- 

 ting microscopic objects so as to reduce the diffraction to a 

 minimum, if not to remove it. In the present case, however, 

 the diffraction is entirely removed. As no light whatever passes 

 by the edges of the fibres, there can be no interference, and 

 consequently no diffraction and no fringes, and therefore the 

 fibre must be seen with the most perfect definition. 



In making these observations, I discovered another advan- 

 tage arising from the employment of polarized light. In many 

 cases we cainiot use a second plate of glass between the object 

 and the object-glass of the microscope, so as to have the object 

 jilaced in a stratum of fluid with parallel sides: we are ihere- 

 tbre obliged to allow the rays from the object to suffer refrac- 

 tion through the irregular surface of the fluid in which it lies. 

 There are cases, too, where the object is imbedded in a re- 

 fracting medium, upon which we can neither grind, if it is 

 solid, nor obtain, when it is fluid, a flat surface. The irregu- 

 larities therefore of surface around the object to be viewed, 

 refract the transmitted light in such a manner as always to 

 injure vision, and often to obliterate the object altogether. 

 By using polarized light we get rid entirely of this irregularly 

 reliacted light, with the exception of that minute portion which 

 corresponds with the surface directly interposed between the 

 object and the object-glass. In order to witness this effect, 

 we have only to place some of the spirals of the CoUomea on 



