210 Transactions of the Royal Microscopical Society. 
only those parts which give the right tint appear black when the 
two plates are properly combined. In other parts the tints are 
raised or lowered, as the case may he, and it may be so difficult to 
distinguish between these higher and lower orders of colours that 
the determination may be very doubtful. 
In applying this method to the microscope, thin flat plates of 
selenite of various thickness have usually been employed, either 
under the object on the stage, or under the analyzer over the eye- 
piece. The chief practical difficulty was to select a plate of selenite 
of such a thickness that its tint with polarized light was so nearly 
the same as that of the crystal under examination, that there could 
be no kind of doubt when the tints were raised and when 
depressed. If the apparatus contains many plates of selenite, 
much time is consumed in finding the right one ; and if it contains 
hut few, none may give such a decided result that the direction of 
the positive and negative axes can be seen at once, and no con- 
sideration be required. 
Now the method which I have lately adopted combines all the 
advantages of a very large number of plates of selenite with the 
practical convenience of a single plate, and also enables us at once 
to determine the true order of the tint given by any crystal under 
examination. I have a wedge-shaped plate of quartz, cut parallel 
to the principal axis, inch long, and ^ inch wide. At its 
thickest ends it is A„th of an inch thick, and thins off to the sharpest 
possible edge. This is fixed on a glass plate so as to leave a space 
of glass tiyths of an inch long by £ inch broad, beyond this thin 
end of the quartz. The combined plates are fixed in a brass 
frame, like that for a micrometer, which slides into the eye- 
piece. On using polarized light with a crossed analyzer over the 
eye-piece, and arranging the plate so that the part with only glass 
is in front, we see the object in its normal state, and the rest of the 
field black, and on pushing forwards the quartz wedge we see 
the field of the microscope crossed with coloured bands, gradually 
rising from the bluish white of the first order, through all the 
brighter orders of colours to the faiut reds and greens, and upwards 
to what cannot be distinguished by the unaided eye from white 
light. If some crystal giving any tint he on the stage of the 
microscope, we can usually see at once whether the tints are 
raised or depressed, by the manner in which it alters the colour of 
the hands; and by pushing the quartz wedge backwards or 
forwards there may be no difficulty in finding the exact place 
where the plate of quartz so exactly neutralizes the action of the 
crystal that it appears black. If this does not occur in any place, 
and, on the contrary, the tints appear to be raised, the eye-piece 
and the plate must be rotated through an angle of 90 J , and the 
requisite place can then be easily found. The plate of quartz 
