386 Transactions of the Society. 
referred, we can visibly separate the coarse structure from the fine 
and view both side by side, and so on. 
It has frequently been emphasised by microscopists how, in the 
ease of diatoms and many other objects, it is very often not only 
what we actually see that has to be taken into account, but the infer- 
ences which we can draw as to the structure, from that which we 
happen to see ; as a familiar instance, I might mention air or oil- 
bubbles in water. And in all such cases where we have to relv more 
or less on inferences, I think it will be found that the colour-disc 
method renders good service ; it is able to play the part, as it were, of 
a special kind of optical analysis. This leads me to another important 
matter, where I am confident that the application of the method will 
yield good results, viz. in theoretical investigations concerning the 
optics of the Microscope, and certain questions in optics generally. 
It appears to me that such questions as the mutual interaction of 
light of different wave-lengths under certain circumstances, the exact 
parts which the different zones of the objective play in the formation 
of the image under varying conditions, the relative quality and nature 
of dioptric and diffraction beams, and other problems relating to inter- 
ference of fight-waves as connected with the formation of the micro- 
scopic image, might be further elucidated with its help. 
To a limited extent I have experimented on the above fines 
myself, but more exhaustive investigations than I am capable of 
carrying out, even if' I had the time to do so, are certainly desirable, 
and I hope that they will be undertaken by some of the Fellows of 
the lfoyal Microscopical Society, or other competent men of science. 
At this place I should like to observe, too, that this paper 
does not profess to be more than a record of experiments and an 
exposition of my personal view s, which, as far as they relate to the 
practical uses of the methods in question, must remain to be verified 
by the actual results of independent workers in the various branches 
of practical microscopy. As for the utility of the method in the 
advancement of microscopical optics, I am glad to say. in confirmation 
of my views, that this has been recognised by such well-known 
authorities as Prof. Abbe and Dr. Czapski, of Jena, who expressed 
to me, about a year ago, their opinion that the method is of much 
theoretical interest in itself, and would probably prove of value in 
theoretical research. 
There is one department of microscopy where the colour-disc3 
may perhaps be used to advantage, to which I have not yet alluded, 
viz. photomicrography. 
This subject is too large to deal with in a paper like the present ; 
moreover, my experiments in this direction are too few to enable me 
to form an opinion as to its practical utility. May be, however, that 
bv its means, for instance, we shall be able to secure greater contrast 
with high-power photography since we are able to modify the relative 
actinic power of the light depicting the background and that depict- 
