*jTuniai,S™risS^T PROGKESS OF MICROSCOPICAL SCIENCE. 277 
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 haphazard 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 
acquainted 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 instrument consists 
of a frame carrying a Nicol'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 movable cells, toothed around their circum- 
ference. At one side of the apparatus there is fixed a metal pillar, 
upon which are arranged three toothed wheels, which only move in 
unison ; 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, that during 
one revolution of the first selenite, the second accomplishes 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 principle 
of a Kellner's eye-piece, the field lens of which receives the whole of 
the polarized 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 depolarizing axes set in any relative position that 
may be desired at starting ; and in order to make every position cer- 
tain, and referrible for reproduction at any after-time, each cell is 
graduated, and reads from an index on its own bearing. The cir- 
cumference 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 
* It is not intended to be conveyed that the rotation of any single selenite 
plate corresponds optically to such a reduction in thickness as to confer upon it 
the chromatic range of the spectrum ; but only to such a reducLion as is competent 
