Chromatic Curves of Microscope Objectives. By E. M. Nelson. 7 
these lenses; my reasons for this are, first, the introduction of apochro- 
matics, and secondly, the use of Jena glass. The conditions which 
obtained before this no longer hold good, for lenses free from colour 
are among the most brilliant and sharp objectives, neither is an orange 
or brick-dust red any longer a bar to fine definition. But as in 
achromatic days we have seen that freedom from colour was not the 
most desirable end to be sought for in a Microscope lens, so too in 
these apochromatic times we must not reject a lens on account of the 
presence of colour. We may well ask, if the removal of colour is not 
the object in an apochromatic, what do we gain by it ? The answer 
is (a) an enlargement of the ratio of aperture to focus, ( b ) an increase 
of brilliancy with equal apertures. Taking (b) first, let us compare 
an ancient but well corrected 2/3 of ’3 N.A. with a modern apochro- 
matic of the same aperture and power. We shall find that the 
apochromatic will give us, first, a more brilliant image on account of 
the more perfect concentration of light, and secondly, a more 
beautiful image on account of the removal of false colour ; but will it 
show us anything more or define anything that we cannot see with 
the old lens?- Rigidly speaking the apochromatic might show the 
flagellum of a bacterium which the old achromatic would fail to do, 
but practically speaking little difference would be observed in this 
respect. Only a dilettante would search for the flagellum of a 
bacterium with a 2/3. Therefore we may say that the difference 
between the apochromatic and the achromatic is merely an aesthetic 
one, viz. that of beauty and brightness of image. With regard to (a) 
however, if we take an apochromatic 1/2 of ’65 N.A. where shall we 
find the old lens with which to compare it? The old 1/2 inches of 
80° were over-powered by 25 to 50 per cent. — in fact they were 4/10 
and 1/3, consequently they are out of court; but suppose we admit 
this largely diminished ratio in aperture, we shall find that a further 
reduction of 20° must be made in the aperture of the old lenses before 
the image is sufficiently cleared from fog to be suitable for comparison, 
assuming of course that full cones are employed in all cases. 
We have here something entirely distinct and new. The micro- 
scopic world had never until the introduction of the apochromatic 
seen such a ratio of aperture to power. The previous alleged examples 
were, as lawyers would say, first, not true in fact, and secondly, if true 
in fact were failures. 
Those observers who use the 1G mm. and 8 mm. apochromatics 
have a more aesthetically perfect lens than was formerly possible, but 
apart from this aestheticism and photography they have nothing that 
was not obtainable under the old conditions, but those who use the 
24 mm. and 12 mm. apochromatics have entirely new conditions which 
were not before practical. 
Until Prof. Abbe invented the apochromatic principle there was 
no method of reducing the spherical aberration for more than one 
