222 J. J. FIELD ON THE RATIO-MICEO-POLARISCOPE. 



that during one revolution of the first selenite, the second accom- 

 plishes two, and the third three. As a matter of convenience a 

 fourth wheel is added, cut with the oblique teeth, needed to gear 

 into a four-threaded driving screw. This latter being the means 

 of giving motion to the whole. 



Over the selenites is placed a condenser, constructed on the prin- 

 ciple of a Kellner's eye-piece, the field lens of which receives the 

 whole of the polarised beam, and converges it upon an achromatic 

 combination, so that no diaphragm being needed, the entire beam 

 passes to the object. 



Lastly, there is an arrangement by which the selenite cells can 

 be instantly ungeared, and turned singly with the finger, so as to 

 have their depolarising axes set in any relative position that may 

 be desired at starting ; and in order to make any position certain, 

 and referable for reproduction at any after time, each cell is gradu- 

 ated, and reads from an index on its own bearing. 



The circumference of the prism collar is also graduated through 

 a certain range, and there is a small projecting stud in the upper 

 part of the apparatus, intended to fit into a corresponding recess to 

 be made in the sub-stage of the microscope, so as to ensure the 

 apparatus occupying on every occasion the same exact position. 

 Thus the whole optical arrangement, when placed for use in the 

 sub-stage, may always be set at zero ; and as a consequence, when 

 once the exact adjustments for developing any particular structures 

 are found, they can be recorded, and instantly reproduced when 

 needed. 



Now, supposing the selenite plates to be so locked in the driving 

 wheels that their positive axes all point to zero, it is clear that on 

 turning the driving screw, so soon as the first selenite begins to 

 move, the second will be gradually parting company with it ; and 

 the third (as to axial relation), will be in advance of both ; and 

 since the rotation of each selenite plate corresponds optically (within 

 the limits of that one plate), to its gradual reduction in thickness ; 

 and all three selenites, starting from zero, can only resume that 

 position after three entire revolutions, during every portion of 

 which they are all occupying different axial relations to one another 

 and to the object ; it is manifest, that the optical effect must be the 

 same as though a great number of different thicknesses of selenite 

 had been tried in succession. 



Thus, in examining any object by polarised light, it is only ne- 



