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



Now, inasmucli as that tlie same agency, whicli can thus deal 

 with pure questions of force, is also competent to reveal to us many 

 of the secrets which nature has locked within those small fairy 

 caskets composing the microscopist's treasures ; in the minute or- 

 ganised cell for instance ; or the almost invisible germ, that 

 may some day marvellously unfold in colour and form 

 of harmony and beauty ; it might well be supposed 

 that the alliance of such an agent as polarized light, 

 with the microscope, would necessarily have led to important dis- 

 coveries in, and optical developments of minute structure, in almost 

 every branch of microscopical enquiry. Yet this has not been the 

 case — I mean to any extent at all commensurate with the powers of 

 either agency singly. I apprehend, however, that the reason of 

 this is rather to be sought in the inefficiency of the means hitherto 

 employed for the application of polarized light in microscopical 

 inquiries, than in any deficiency of power in the combination 

 itself ; and I have good reason to believe that the instrument 

 which I shall presently have the honour of describing ; and which I 

 have named (as in some measure indicating its nature) the Ratio- 

 Micro-Polariscope; will be found to place polarized Ught in a more 

 important position than it has hitherto occupied as an agent in 

 microscopical research. 



Knowing that I have the honour of addressing many gentlemen 

 who are perfectly familiar with optical science, I wish to express 

 the hope, that I may be pardoned for prefacing what I have to say 

 with a few elementary remarks, relative to the changes that occur 

 in a polarized beam when it suffers double refraction. I feel obliged 

 to do this in order to make the subject generallu intelligible ; but I 

 will endeavour to be as brief as possible ; and having thus (to the 

 best of my ability) made the principle of the instrument clear ; I 

 will proceed to describe its mechanical details, and its modes of 

 use and action. 



When a beam of polarized light passes through any substance 

 possessing the optical power of double refraction, (and I may re- 

 mark that a much larger number than is usually supposed of the 

 structures falling under our observation do possess this power) it 

 is split into two distinct rays, polarized in the same plane. The 

 ray emerging in a direct line with the original beam is called the 

 ordinary ray ; whilst that which deviates from the direct course is 

 known as the extraordinary ray ; and these two are not only separ- 



