54 
PRACTICAL PHOTO-MICROGRAPHY. 
in which the actinic and visual foci should be coinci¬ 
dent. At first, this was only imperfectly achieved, for, 
with the samples of glass then obtainable, it was 
impossible to correct objectives so as to make all the 
spectrum rays coincide in focal point; and objectives 
would be corrected for only two sets of rays. 
Let us suppose that a given objective was corrected 
so that the blue and yellow rays should come to focus 
in the same plane vertical to the optical axis ; then the 
red and green rays would still focus on other planes, 
and this fact would be demonstrated by the existence 
of coloured fringes round the image, the. fringes in this 
particular case being red and green to the eye, and 
represented on the photographic plate by more or less 
blur. In spite of this defect many of the objectives 
made just before the introduction of “ apochromatic ” 
objectives gave good results in photo-micrography, but 
never such results as are obtained now with apochro¬ 
matic glasses, or even with achromatic glasses on ortho- 
chromatic plates. 
Experiments conducted in Germany by Dr. Abbe 
and his colleagues, and aided in the most laudable 
manner by the German Government, resulted in the 
production of many samples of glass “ metal,” some of 
which have such qualities of density and dispersion that 
Abbe was able to calculate objectives to be made of 
some of these glasses, wherein corrections can be made 
so as to practically bring the entire visible spectrum to 
one focal point. Technically, Abbe was able by objec¬ 
tive and ocular combined to eliminate the whole of the 
