On Colour-Illumination. By J. Bheinberg. 
143 
the other part, the red part of the disc needs to have given to it the 
special opportunity of affecting the eye in the strongest possible way ; 
therefore the central portion of the disc must he assigned to this 
colour. Vice versa I have not been able to obtain satisfactory results. 
“ I may here be permitted to say a few words as to the colours 
which I have found most suitable for discs for the other kinds of 
illumination. 
“ As regards the low-power method, it is essential for good results 
to keep the central portion of the disc of a colour not too luminous. 
On this account blue and green are preferable to red, and the most 
generally useful disc appears to be a malachite-green spot, an 
“ Oxford ” blue spot, and an olive-green. If a red ground is used, 
care must be taken to lessen its brilliancy by narrowing the band of 
the spectrum which it passes. This can be done by superposing a 
green ; as almost all greens will, curiously enough, be found to trans- 
mit a certain range of rays in the red portion of the spectrum. It 
will be found in general to be a much more convenient way of getting 
at a good colour for central spots, to superpose different colours, and 
get at the result by subtraction as it were, than by adding two or 
three layers of the same colour. For the peripheral portion of those 
discs in which two colours are employed, red or orange will be found 
most useful, though almost any colour, no matter whether a pure or 
a mixed spectrum colour, will do, so long as it is sufficiently luminous 
relatively to the central spot. 
“As regards the choice of discs, it will be found that opaque and 
thick sections are better seen when a disc is used having both rim 
and centre coloured ; in fact there is a striking difference in the clear- 
ness of such objects as seen with a double coloured disc compared 
with their appearance when discs with centre coloured only are used. 
On the other hand, the latter kind of disc is usually preferable for 
diatoms and thin sections. 
“ Now, with regard to the preparation of discs for use above the 
objective in the high-power or diffraction method, the colours used 
for the central spot should not be too dark. It is essential for the 
most perfect performance of the disc that its central spot should allow 
a certain range of colours to pass through it common to the peri- 
pheral portion. By this means the untoward diffraction effects seen 
when the dioptric rays are stopped out with a black spot, are evaded 
and avoided. A light blue or a light green, which, as is well known, 
usually pass a wide range of the other colours of the spectrum, are to 
he most recommended for the central spot ; a red should only be 
employed in the centre, when the peripheral portion of the disc is left 
uncoloured. 
“ With regard to the choice of discs for viewing particular objects, 
those with a red periphery, having the centre either coloured light 
blue or left uncoloured, are most suitable for relatively opaque and 
thick sections, whilst discs with a blue rim are best where it is a case 
