10 
Transactions of the Society. 
14 per cent, of increase of resolving power with medium and lower 
apertures, but with * 95 N. A. or over, the effect is greatly lessened 
(see Table). This being the case, why should monochromatic light 
be so serviceable, especially with wide-angled achromatics ? We have 
seen that the presence of colour in an achromatic is no bar to defini- 
tion and resolution. I have seen many old high-power achromatics 
which will show anything that apochromatics will. I believe Dr. 
Dallinger still has in his possession some old achromatics that will do 
so (the visibility of minute flagella, which probably depends on what 
we have termed the aestheticism of the image, should perhaps be 
excepted). It is the removal of the spherical aberration due to 
differences of colour caused by diffraction. This statement may not 
be very clear ; an example will perhaps explain it. Let us examine 
an ordinary P. angulatum with a cheap 1/7 of *8 N.A., the image 
will be what might have been expected ; but under monochromatic 
illumination, even when that is not of a short wave-length, we obtain 
what we should not have expected, viz. a wonderfully sharp image, a 
something more than the mere removal of the outstanding colour ; in 
brief, you would think that you were examining the object with a 
very wide-angled lens, but at the same time you would not notice 
any increase of resolving power. What I mean is that you would 
see none of the finer detail of the object which could only be resolved 
by a very wide angle, but the detail that you do see would appear as 
if it were being shown by a very wide-angled lens. These appearances 
are familiar to those who have worked with monochromatic light. 
Let us now consider the conditions we have when the object is seen 
with ordinary light, and say a rather small cone of illumination. 
With the lens of ' 8 N.A., the six green spectra of the P. angulatum 
would appear in the peripheral zone of the objective, and the white 
dioptric beam would be seen in the centre of the lens. Therefore, 
we have by means of diffraction practically a monochromatic illumi- 
nation in the outer peripheral zone of the lens (the red and orange 
being cut off at the edge of the lens, while the blue and green are left, 
but the blue is too weak to affect the image), and in the centre of the 
lens white light. 
Now the perfection of the image consists in the exact union of 
these outer spectral beams with the central beam. But this exact 
union probably does not take place, because the spherical aberration 
of the objective is very likely not corrected for these two colours, 
viz. the green in the peripheral zone and that which most strongly 
affects the eye, viz. tbe orange-yellow in the centre. The moment 
monochromatic light is used the centre is made of the same colour as 
the periphery, and if that colour for which the lens is corrected be 
chosen, a magnificent image is the result. Those objects which bring 
the first order spectra well within the grip of the lens are not so 
much affected by monochromatic light, but it is the finer detail, the 
spectra of which would be at the periphery of the lens, which is 
