J. J. FIELD ON THE RATIO-MlCRO-rOLARISOOPE. 221 



of action theoretically, I have reason to place great confidence in 

 its capabilities ; yet, at the same time, I am very solicitous that 

 no exaggerated ideas respecting it should be entertained. It must 

 be remembered that the defining and penetrating powers of micros- 

 copic objectives, have been devcloj)ed in so wonderful a degree, that 

 comparatively few structures can altogether escape them ; never- 

 theless, there are often many details which appear but indistinctly, 

 even under the best glasses, and the most improved methods of 

 illumination by common light ; and all of these (possessing a doubly 

 refracting structure) can certainly be brought out with marvellous 

 force and distinctness by some particular setting of this Polaris- 

 cope. 



On the other hand, no microscopist need look very far through 

 his collection, before meeting with certain structures that altogether 

 refuse to be evidenced without polarisation ; but in such cases even 

 polarised Ught is of little avail, unless certain exact conditions, or 

 at all events, a very near approximation to such exact conditions, 

 of the polarised beam in relation to those structures can be com- 

 manded. Indeed I have repeatedly observed that when polarised 

 light is employed in a hap-hazard manner, it may indeed paint the 

 object with gorgeous hues ; but instead of developing, it too often 

 optically obliterates detail. 



It is the aim of the instrument — the construction of which I shall 

 now describe — to displace this hap-hazard mode of operating, and 

 enable microscopists to mete out to each particular structure its own 

 special needs and requirements. 



This is what I claim for it, and I believe that in the hands 

 of those who will take the pains to become practically ac- 

 quainted with its capacities, and have patience to vary its combina- 

 tions until the correct ones be found, it will prove a valuable aid, 

 not only in study, but also in original research. 



The instiument consists of a frame carrying a Nichol's prism 

 and three plates of selenite. The prism is arranged in a rotating 

 collar, and the selenite plates above the prism are fitted into move- 

 able cells, toothed around their circumference. 



At one side of the apparatus there is fixed a metal pillar, upon 

 which are arranged three toothed wheels, which only move in uni- 

 son ; and the toothed selenite cells are so arranged as to size, that 

 they gear into these pillar wheels, and take motion from them ; 

 whilst, at the same time, the relation between the wheels is such, 



