278 PROGRESS OF MICROSCOPICAL SCIENCE. [SSilSriS 
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 polarized light, it is 
only necessary to make three entire turns of the driving-wheels ; and 
then, if the exact selenite supplement needed for developing the struc- 
ture be within the compass of the zero setting, some position must be 
arrived at in which the details sought may be made to appear with the 
greatest possible distinctness. Should not this be the case, the selenite 
cells are to be ungeared, the plates re~set, with their axes more or less 
out of coincidence, and the former operation repeated, and so on. It 
will thus be seen that the number of variations this instrument is 
capable of producing — variations that may be so conducted as to 
create a gradually increasing or diminishing effect in tJie direction that 
appears to he needed, and which variations, starting as they do from 
known data and proceeding in known ratio, can always be reproduced 
at will, are almost endless. The ratio movement, by spreading out 
the depolarizing axes of the selenites in a predetermined order (some- 
thing after the fashion of the opening of a fan), enables the observer 
rapidly to arrive at an approximation to the most perfect optical con- 
ditions for viewing any particular structure ; and then, in order to 
arrive at absolute perfection in the development of the details, nothing 
is needed but a little time and patience, to change the setting, tooth 
by tooth (in the direction indicated by the previous adjustments), until 
further change becomes detrimental. 
By a very simple notation, fine positions can be instantly recorded 
and afterwards read at a glance ; and although many trials are gene- 
rally needed to arrive at the finest effects, still, when any adjustment 
giving superlative results with any special object is found, such adjust- 
to dimmish the intensity of its effect from the maximum, gradually onwards to 
the minimum, until one of the neutral axes is reached. But in this instrument 
there are three selenites, of such substance that in a certain set position, in which 
each plate is individually active in the production of colour, white light results, in 
consequence of the chromatic interferences mutually neutralizing one another. It 
is therefore clear that the least motion of the driving-screw will, in the most 
gradual and micrometrical manner, upset this balance of colour, by diminishing 
the power of a more than b, and of b more than c, and by continuing the 
motion, this change in the proportion of the three colours composing the depolarized 
beam will proceed in the same direction, and in the same ratio, until one of the 
neutral axes is reached, in which position a becomes at once inoperative. On again 
driving the selenites, a begins to increase in intensity whilst b is diminishing ; 
and soon another neutral axis being reached, b, in its turn, becomes inoperative, 
and then having passed the neutral position, proceeds onwards, with gradually 
augmenting power, to reinforce a, and so on. It wall be seen therefore that the 
rotation of the selenites, as here effected, corresponds in the fullest sense to gradual 
reduction in thickness ; and I may compare the depolarizing effect so produced to 
that of passing beneath the object a wedge of selenite, of micrometrically small 
angle and of infinite length ; and tlie chromatic results to those obtained by an- 
artist, who, having the primary colours, a, b, and c on his palette, has the power 
of combining them in all proportions, and skilfully mingles them according to his 
wants. 
