Neiv Way of producing Colour-Contrast. By J. Bheinlerg. 375 
But, along with the increase of definition due to this cause, there is 
unfortunately a not inconsiderable loss of contrast, because the object 
appears to he more or less of the same colour as the background. 
To photomicrography the above applies also — perhaps I ought to 
say especially. 
It will be part of my task this evening to show you subsequently, 
that by means of the colour-disc method the advantages of the mono- 
chromatic illumination can in a great measure be retained, whilst its 
drawback is to a great extent obviated. 
Secondly may be mentioned the poJariscope in conjunction with 
selenites, &c., which, when used for its proper purpose of investigating 
the crystalline formation of objects, &c., is in nowise related to the 
methods under consideration. But whilst in their scientific applica- 
tion both methods have their distinct aims and results, they are 
undoubted rivals when compared as agents for the production of 
striking colour effects, and for this rather subordinate purpose the 
colour-disc method may claim the further advantage of greater 
simplicity. 
And, lastly, I must mention dark-ground illumination as affording 
a means of changing the colour of the background ; but from this 
I will at once pass on to the colour-disc method of illumination, since 
one part of it indeed ought, strictly speaking, to be considered as 
an extension of the principles of ordinary dark-ground illumination. 
I say one part of it, for it is necessary to separate the colour-disc 
method of illumination into two distinct parts, the one applicable 
chiefly to high and the other to low powers. At first sight these two 
methods may appear to be almost opposed to one another, but upon 
closer examination it will be evident that there is a common principle 
underlying both. 
In the first case, an objective of wide aperture is used in conjunc- 
tion with a comparatively narrow cone of light from the condenser. 
This method is chiefly of use with high powers. 
In the second case, an objective of small aperture is employed, 
together with a wide cone of light from the condenser, just as with 
fiark-ground illumination, and this method is for use with low 
powers. 
The common principle underlying both is that, compared with 
ordinary illumination, we allow a much smaller proportion of direct 
light to enter the objective (i. e. light whose direction is wholly 
determined by the action of the optical system), relatively to the 
quantity of indirect light (by which I mean light which is taken up 
by the objective after having suffered refraction or diffraction by the 
object). 
We must consider this fact and what it entails a little more in 
detail. 
The Abbe theory of microscopic vision has taught us the respec- 
tive roles played by the illuminating cone and the objective cone. 
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