218 J. J. FIELD ON THE EATIO-MICRO-POLARISCOPE. 



refer to afford very decided chromatic effects, provided a plate of 

 some depolarizing substance (preferably selenite) of such thickness 

 as in itself to produce colour, be interposed either above or below 

 the object ; the details in this case being mapped out (so to speak) 

 by their becoming of a different tint from that of the general 

 field ; but as, in order to develope any particular structure in per- 

 fection, a selenite plate of a suitable relative thickness must be 

 emj)loyed ; and since the various tissues that come under observa- 

 tion present innumerable parts, differing in thickness, and in power 

 of refraction ; it becomes evident that for the perfect development 

 of microscopic details any single selenite possesses but a very 

 limited range. Hence microscopists are generally supplied with 

 three or more selenite j)lates ; and inasmuch as that the power of 

 any such plate can be made to vary (within certain limits) by rota- 

 tion, the selenites are frequently mounted in cells that can be 

 turned with the finger for that purpose. 



The question now arises, how happens it, that such a large 

 number of structures, which give no evidence of double refraction, 

 when viewed between prisms alone ; furnish very decided evidences 

 of it when selenite plates are superadded ? I have never met 

 with any answer to this question ; and if I venture to give an 

 explanation myself, it is only because I cannot otherwise convey 

 what my ideas were in designing this polariscope ; or show in 

 what manner it is capable of accomphshing what I assume to be 

 needed. 



A beam of white light, being composed of a number of rays of 

 unequal wave-lengths, and therefore of unequal refrangibilities, it 

 follows, as before stated, that if there be produced interference 

 between two such beams, when the two differ in phase by any 

 amount short of a semi-undulation, such interference must result 

 in the suppression of a p>ortion only of the constituents of the 

 beams, leaving the remainder in great measure unaffected. 



But as a beam of ordinary light contains, besides the various 

 coloured or visual rays, certain others, which are of less refrangi- 

 hility than any visual ray — these being the heat rays extending 

 beyond the red ; and again, at the other end of the spectrum, 

 others, of greater refrangihility than any visual ray, viz., the 

 chemical rays, extending beyond the violet ; it follows that if the 

 relative phase-displacement of two beams were either so slight as 

 to be only a small fraction of a semi-undulation ; or on the other 



