220 J. J. FIELD ON THE RATIO-MICRO-POLARISCOPE. 



ing structures — or in portions of doubly -refracting structures of a 

 composite order, such as sections of wood, where there are many 

 varying thicknesses, and many differing tissues — the retardation of 

 the extraordinary ray is either so small on the one hand, or so 

 large on the other, that the effective clashing, upon revolving the 

 analyser — speaking relatively to many details, and not as to the 

 entire section — is confined to the two extremes of the spectrum, 

 i.e. the chemical and calorific rays. 



But it is evident that the major displacement of phase, resulting 

 in the destruction of the heat rays ; or the minor displacement, 

 resulting in the aimihilation of the chemical rays, may each be so 

 supplemented by a definite thickness of selenite, as to have their 

 influence (previously imperceptible), carried forward into the region 

 of colour. The power of the former being so supplemented as to 

 pass the first semi-undulation, and interfere chromatically in the 

 second ; whilst that of the latter may, in like manner, be advanced 

 in the first semi-undulation, producing similar results ; and thus, 

 both the greater and lesser phase-displacements (of which there 

 was no optical evidence whatever before), become plainly visible by 

 change of tint. 



But, inasmuch as that the whole spectrum, as to wave-length, is a 

 graduated amount — one part merging into another ; every length 

 of wave, in fact, existing between the two extreme components of 

 a beam of white light — it follows that in oxdiQx perfectly to supple- 

 ment the innumerable thicknesses of doubly refracting structures 

 that the microscopist meets with, thicknesses of selenite equally 

 innumerable would be needed, unless something equivalent to these 

 innumerable thicknesses could be devised. 



I am happy to say that the Polariscope I am now about to des- 

 cribe, although not presenting these innumerable thicknesses 

 actually, nevertheless does so virtually ; and may be made to sup- 

 plement any thickness of any doubly refracting structure whatever, 

 and thus exhibit it with the fullest possible efficiency and force. 



The instrument admits of such a vast number of combinations 

 that its full powers cannot be at once ascertained ; but in my hands 

 its operation has been already most beautiful and strilring, and by 

 its means I have developed (optically), in the plainest manner, 

 structures which before were too faint and indistinct to be other- 

 wise than questionable. 



Speaking not only from this experience, but also from its mode 



