388 Transactions of the Society. 
the use of a separate lens for each colour effect, one of the objective 
lenses itself might be suitably coloured, but even this would have the 
disadvantage that the size of the spot would be fixed, whilst practice 
shows that larger spots are desirable for some objects than for others. 
No doubt, however, opticians can find some simple and convenient 
arrangement in order to be able to use the discs in best harmony with 
the rest of the Microscope. 
With the low-power method it is desirable to have an arrange- 
ment for altering the distance between the discs and the back lens of 
the condenser, within certain limits, as this will allow us to control 
the N.A. of the cone passing through the central area as necessity 
may require. 
I shall not trouble you much longer, as this paper has already 
exceeded in length what I intended, but I cannot close without a word 
or two as to an objection which will probably be raised against the 
use of the high-power colour-disc method. 
At a time like the present, when there are two schools of micro- 
scopists, the one in favour of wide, the other in favour of narrow cones 
of illumination, I am fully aware that any method adapted for work 
in the latter manner will be looked upon with disfavour by many. 
Personally I cannot make up my mind in favour of either of the two 
parties, the subject is too intricate ; there are too many pros and cons 
on both sides ; and when we see the war waging on the subject in the 
4 English Mechanic * as recently as this year, and come to note the 
diversity of opinion of the various authorities, I would submit that the 
whole question must await the light of further investigations before 
we can definitely consider the matter settled, and that at present it is 
to a great extent an empirical matter what size cone of illumination it 
is best to use. Perhaps the colour-disc method will contribute to the 
further elucidation of this most important problem. 
In conclusion, I will express the hope that the methods of observa- 
tion discussed this evening will be taken up and developed by opticians 
and practical microscopists, and that before very long they will be 
definitely established as an aid to microscopical research. 
