5 
II . — The Chromatic Curves of Microscope Objectives . 
By E. M. Nelson, F.R.M.S. 
( Read 15 th February , 1893.) 
The achromatism of the Microscope objective is a subject which has 
received comparatively little attention except from the practical con- 
structors of Microscope lenses. It is barely mentioned either in the 
text-books or in the journals of our Societies. The only work in our 
language, so far as I am aware, dealing with it at all at length is the 
English translation of Naegeli and Schwendener. The reason for this 
is not far to seek : excellent lenses often yield strongly coloured images, 
and lenses giving colourless images are usually defective in more 
important points. The presence or absence of colour is by itself no 
criterion of the excellence of either the Microscope or telescope 
objective. There are other points of greater importance which are 
liable to be overlooked. There is no question of colour correction in 
a reflecting telescope, its excellence depends on figure and centering, 
and these points are of equal importance in the dioptric telescope. 
Now, spherical aberration is much more obtrusive in Microscope 
than in telescope objectives, because their apertures are vastly greater. 
An aperture of *0625 N.A. is considered remarkable for a dioptric 
telescope, the usual aperture is half that amount. The new Greenwich 
telescope is to have ’0416 N.A., the greatest yet given to a large 
instrument. In achromatic days, as hinted above, objectives free 
from colour were as a rule to be avoided, because this freedom from 
colour was usually obtained at the expense of sharpness. There were 
two kinds of correction in vogue with achromatic lenses : one (A) left 
the image decidedly blue, and the other (B) red, and many excellent 
objectives were constructed on both these principles. The red kind 
of correction (B) must be divided into at least two, if not three, sub- 
divisions, for there was (B i) an orange red which was bad, as a 
successful example of a lens exhibiting that colour of image had not 
been seen ; all the fine glasses of the (B) type gave purple red (usually 
called claret) coloured images, and of these there were two groups, 
(B ii) the bluish purples, and (B iii) the reddish purples. Perhaps 
the finest glasses of old construction were those which gave images 
nearly resembling a solution of permanganate of potass and water, or 
the more homely damson-juice, in colour (B ii) and (B iii). 
In selecting an objective the great desiderata were sharp and 
brilliant images, and these desiderata were usually accompanied by 
violent colours. It will be said that these colours are those of the 
secondary spectrum due to the irrationality of ordinary crowns and 
flints, but I think you will agree with me that there is something 
more than this when you exnniine some of the curves presently. You 
will notice that I have employed the past tense when speaking about 
