s 
colours of the spectrum, and the quality of the projected image is, therefore, within 
wide limits independent of the nature of the illuminating source, which may be 
white or compound, or monochromatic light emerging from any section of the 
spectrum. 
In the older series again complete colour correction is obtained for 
one zone of the objective only, a marked deterioration being observable towards 
the margin and the centre of the aperture, whilst in the apochromatic lenses it is 
corrected uniformly for all zones. Consequently, in using Abbe's test-plate, scarcely 
more colour is perceived with the most oblique illumination than with central light. 
Finall}^, even within the zone of the most complete colour correction of the 
ordinary achromatics, only two colours can be united in one point. The various 
coloured images therefore can only fall on the same spot in pairs between which 
there is a considerable difference in focus. In the new series, however, three 
colours are brought to a focus, whereby the amount of focal difference for the 
various sections of the spectrum, from the visible to far into the chemically active 
portion, is reduced to '/^ to i/^^, of its original magnitude i. e. practically 
eliminated, and this, as has been stated before, equally for each zone of the 
objective. The images due to each single colour, thus individually corrected, are 
rendered coincident und collectively form the final image i). 
The practical advantages of these new lenses become at once apparent. 
A considerably increased concentration of light with ordinary eye -piece observ- 
ation or any other mode of application under all possible conditions of illumination 
confers on these glasses an acknowledged superiority over the ordinary achromatic 
lenses botli witli respect to optical capacity and diversity of applicability. 
The natural colours of objects, even in the more delicate tints, are 
truly reproduced by these objectives. Close to the margin of the field, 
the images are nearly as sharp as in the centre, though tlie high aperture 
and the relatively great working distance render a moderate degree of curvature 
of the image unavoidable in these objectives as in the older ones; the central 
1) It would be a grave error, both theoretically and practically, to view this 
process of achromatisation of a higher order as here defined in the light of 
a mere improvement of ordinary "achromatism", such as would result from 
a diminution of the secondary spectrum while yet only two colours are united, or 
from achromatisation embodying the principles here indicated but limited to one 
particular zone of the objective (which might be obtained by the introduction of 
suitable glasses into objectives constructed after the usual formula). 
The word "apochromatic" was introduced by Prof Abbe as a technical term 
for this other kind of achromatism, long familiar to scientists as a theoretical 
idea but only recently realised practically. With deference to intelligible technical 
phraseology it is desirable that this expression thus clearly defined by its author 
should retain its original meaning, and any attempt to utilise it for 
jiur poses of trade advertisements should be discountenanced. 
