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Transactions of the Society . 
Again, if we use a disc with centre red and the remainder bilat- 
erally divided (plate IX. fig. 1 e ) and look at a serrated or fibrous object, 
e.g. a bundle of muscle- fibres, the one side of the separate fibres 
appears blue, the other yellow. The general form and structure of 
an object are brought out most forcibly in this way. 
The same effects which can be obtained with low powers by use 
of coloured discs in the diaphragm-carrier of the condenser may also 
be obtained, though not so conveniently generally, by placing screens, 
coloured in similar fashion to the disc3, between the source of illumi- 
nation and the mirror, and with high powers we can get the same 
effects as those before described with discs over the objective, by 
interposing suitably sized discs between object and objective, or 
between the lenses of the objective — in fact I have obtained my best 
results with the latter method, though for general practice it has very 
obvious disadvantages. As to having the discs between object and 
objective, that has the disadvantage that the spots or other colouring 
on the discs have to be exceedingly minute, and that a disc in this 
position disturbs the correction of the lens very much more than 
when the disc is placed above the objective, so that for ordinary 
purposes the latter plan is most useful. 
It is also possible to combine the high- and low-power methods 
with an objective of * 6 N.A. for example, although I have not yet 
been able to get very sitisfactory results in this way. We should 
in this case use, say, a disc in the carrier of the condenser having a 
blue margin admitting the whole annular cone of light (if I may use 
such an expression) greater than *6 N. A., whilst the remainder of the 
disc would be blackened with exception of a small central space 
admitting a cone of • 1 to *2 N.A. which would remain clear (plate IX. 
fig. 1 h). In conjunction with this we should use one of the ordinary 
discs (plate IX. fig. I c or b) above the objective as described before, 
and the two discs would then be helping each other as far as the 
effect is concerned. 
Now with respect to the applications to which I believe these 
methods of viewing objects will lend themselves. 
In the first place, we can view many unstained botanical and 
physiological preparations to greater advantage. The effect of the 
colour-discs on these in many cases is not only to make the structure 
appear more distinct, but also to increase depth perception, and throw 
the object up into almost stereoscopic relief. This seems partly to 
be due to the mingling of the colour of the background with that 
of the object in a greater or less proportion according to its shape, 
thickness, Ac., which results in a gradual transition between two 
contrasting colours, which, similar to shadow effects, the eye readily 
and unconsciously interprets into the forms of relief. We have in 
fact an intensified form of the well-known stereoscopic effect when we 
look at complementary colours in apposition. 
But apart from this we actually get, for reasons which I cannot 
