THE SPECTRUM-MICROSCOPE. 
71 
tlie apparent magnitude of tlie effect of an increase in thick- 
ness is far greater when the object is thin than when thick, 
and past a certain thickness the change is comparatively very 
slight. If only small crystals can be obtained, it is well to 
mount a number of different thickness ; but when it is possible 
to obtain crystals of sufficient size, it is far better to make 
them into wedge-shaped objects, since then the effect of gradual 
change in thickness can easily be observed. Different kinds 
of crystals require different treatment, but as a general rule I 
find that it is best to grind them on moderately soft Water-of- 
Ayr stone with a small quantity of water, which soon becomes 
a saturated solution, and then to polish them with a little 
rouge spread on paper laid over a flat surface, or else in some 
cases to dissolve off a thin layer by carefully rubbing the 
crystal on moist blotting-paper until the scratches are removed. 
Then, whenever it is admissible, I mount the crystal on a 
glass, and ^lso cover it with a piece of thin glass with Canada 
balsam. Strongly coloured solutions may be examined in test 
tubes, or may be kept sealed up in small bottles made out 
of glass tubes, the light then examined being that which passes 
through the centre of the tube from side to side. Such tubes 
may be laid on the ordinary stage, or held on the stage 
attached to the eye-piece. Smaller quantities may be ex- 
amined in cells cut out of thick glass tubes, one side being 
fixed to the ordinary glass with Canada balsam, like a micro- 
scopic object, and the other covered with thin glass, which 
readily holds on by capillary attraction, or may be cemented 
fast with gold size or Canada balsam, if it be desirable to keep 
it as a permanent object. Such tubes may be made of any 
length that may be required for very slightly- coloured solutions. 
Cells made out of spirit thermometer tubes, so as to be about 
•one-tenth of an inch in diameter, and half an inch long, are 
very suitable for the examination of very small quantities; 
but where plenty of material can be obtained, it is far better 
to use cells cut out of strong tube, having an interior diameter 
•of about three-fourths of an inch, cut wedge-shape, so that the 
thickness of the solution may be one-fourth of an inch, or 
more, on one side, and not above one-fortieth on the other ; 
and then the effect of different thicknesses can easily be 
-ascertained. The accompanying figure will better explain my 
meaning (see fig. 3). If we place a small object on the stage 
of the microscope, and then one of these oblique cells on the 
stage attached to the eye-piece, we can easily move it in front 
of the opening / (fig. 1), until the two spectra are exactly the 
same, or until we can see that they are in no case identical. 
For the same reason, it is very desirable to have pieces of 
•coloured pot-metal glass cut wedge-shape, so as to be able to 
