8 
Transactions of the Society. 
colour. Now as spherical aberration is greatly increased by reducing 
the focus while the aperture remains the same, objectives which had a 
high ratio of aperture to focus always gave images bathed in the 
coloured fog of the unreduced spherical aberration. 
The whole history of the progress of optics may be termed that 
of the increase of the ratio of the aperture to focus. 
Huyghens and Campani’s telescopes had a N.A. of *003, but the 
new equatorial at Greenwich will have •0416 N.A., the greatest yet 
accomplished in a large dioptric instrument. 
The great Dollond, however, made a small telescope of *069 N.A. 
In reflecting telescopes, as there is no spherical aberration to 
correct for dispersion, a very high ratio of aperture to focus has been 
reached. In Gregorians, an aperture as high as • 24 N.A. has been 
attained, and several of *164 N.A. have been made, which gave 
excellent results. 
Newtonians have been made of *083 N.A., and those of *0625 
N.A. are quite common. We see, therefore, that a ratio of aperture 
to focus, which is not in the least out of the way in a reflector, 
is exceptional in a refractor. This digression with regard to tele- 
scopes is useful, because it emphasizes the fact that it is the unreduced 
spherical aberration owing to dispersion which has hitherto barred the 
way to the increase in the ratio of aperture to power, but which has 
been greatly lessened by Prof. Abbe’s apochromatic system. 
But to return to our subject. The statement that objects are seen 
with apochromatic lenses in their natural colours may become mis- 
leading, because the colours of microscopic objects depend largely on 
diffraction effects which are quite dissipated when magnifying power 
is used. The magnificent colour of a scale of a Morpho Menelaus 
quickly disappears with magnification, so does the colour of the 
resplendent diatom Actinocyclus Elirenbergii alter and become less as 
the power is increased, quite independently of the chromatic aberra- 
tion of the lens ; whereas some of the finest apochromatics impart 
faint rose or pale brown colours to colourless objects. We now come 
to the practical investigation of the chromatic aberration of Micro- 
scope lenses. The method employed by Messrs. Naegeli and Schwen- 
dener is the usual one of covering up half the aperture of the 
objective, the remaining half of the lens acting as a prism. They 
also refer to Prof. Abbe’s plan of passing oblique beams through the 
objective by means of stops placed at the back of the condenser. On 
experimenting with both these methods I find that half the objective 
plan is ineffectual, and the drawback with regard to the Abbe method 
was the great amount of spherical aberration in the condenser which 
prevented the central and marginal cones of light being focused on 
the object at the same instant. Far better results can, however, be 
now obtained by using Powell and Lealand’s fluorite apochromatic 
condenser, owing to its aplanatism. But the method for your investi- 
gation to-night differs from either of these. By means of my 
