11 
become light, and if the rotation be continued the light and dark 
fields will alternate at every 90 degrees. When this is the case 
it is clear that the light has been polarised in its passage through 
the first prism. The well known phenomena of colour produced 
by the action of crystallised bodies on polarised light are described 
by Brewster as being ‘‘ the most splendid within the whole range 
of optics.”. The production of those beautiful colours in some 
objects, and with which we are all more or less familiar, is brought 
about by the interposition of a single plate of selenite between the 
polariser and the object which is the form of apparatus most 
common in use. Other arrangements were described, such as that 
of “ Darker’s,” in which three selenite plates are mounted in such 
a way that either one, two, or three can be employed at pleasure, 
each being capable of independent rotation, but without any 
mechanical contrivance for so doing, and by combining these 
selenites in various ways numerous colours together with their 
complimentary tints can be produced. ‘‘ Hislop,” whose labours 
among microscopic crystals suggested to him the necessity of 
having some mechanical means by which the adjustment of the 
selenites as well as their rotation could be effected, contrived what 
he has called his ‘Analysing Selenite Stage,” in which the 
selenites can he mechanically rotated either singly or in combina- 
tion, but at the same speed. This plan is most undcubtedly an 
improvement upon Darker’s. But shortly after this came Field, 
with his ‘‘ Ratio-Micro-Polariscope,” so called from the fact of the 
selenites moving in a fixed ratio to one another; his aim being to 
reduce this mode of observation to something like a systematic 
method, and which his apparatus is well calculated to do; but as 
it is very elaborate and costly, and therefore beyond the reach of 
ordinary observers it is not likely ever to come into frequent use. 
The last arrangement to which I will call your attention is that 
which was the more immediate object of this paper. It was 
devised by Mr. Carl Becker, who is well known for his great skill 
in designing and constructing instrumental appliances for physical 
research, after having seen the much more complicated one of 
Field’s. It combines the chief advantages of the latter with 
extreme simplicity and compactness. The selenites in this 
arrangement are each mounted in a circular toothed ring of the 
same diameter and number of teeth, and which are graduated 
and numbered on the horizontal faces in order to enable 
the observer to set the axes of the selenites in any desired 
position ; these are all enclosed in a cylindrical metal box in which 
they revolve with perfect freedom. One end of this box ter- 
minates in a fitting for the under stage of the microscope, whilst 
the other carries a Nicol’s prism which has an independent rotary 
motion. There are two openings on opposite sides of this box, 
one for adjusting the selenites, and the other to receive a differentia } 
