Tue Microscope. 109 
Another drawback lies in the question whether manufacturers 
will universally adopt the standards, as it will entail a loss in 
proportion to the amount of business, which will in no case be 
small. 
In passing to objectives I will state what is probably known 
to all, that their nomenclature of rating refers to their magnify- 
ing power, and has no reference to working distance. They are 
rated according to a lens, the magnifying power of which is 
equal to that formed by the lenses comprising the objectives. 
Objectives are made up of systems—systems meaning a 
single lens, or a set of lenses, which being cemented together, 
act like a single lens. Angular aperture is the angle formed by 
the extreme rays, which, coming from one point, are transmitted 
by the objectivé. Besides achromatism, it is the angular aper- 
ture of an objective which determines its quality. This it is 
which gives defining or resolving power to an objective,—which 
shows structure and detail. Angular aperture and working 
distance do not stand in a direct relation to each other, as is 
sometimes supposed. They vary according to the construction 
of the objective. In two objectives of the same angular aper- 
ture the working distance is not necessarily alike: even with 
two objectives of the same power and different angle, the one 
with the larger angle may have the greater working distance. 
This brings me to speak of a curious fact which, although 
I was aware of it, did not draw my particular notice until a re- 
cent occurrence. We had an objective, which, to all appearance, 
was in every respect perfect. It was made up of seven lenses, 
gave aclear cut, colorless image, and had good resolution. I 
was, however, not satisfied with it in this latter respect, and de- 
cided to made’a new one. The formula involved only five 
lenses. After the objective had been completed, comparisons 
were made, and with the following result: The angular aper- 
ture was to a degree the same in both. Working distance did 
not vary 1-100th of an inch. Both were equally well corrected, 
and had the same magnifying power; and still: the new one, 
with only five lenses, resolved 60,000 lines to the inch, where 
the old one with seven lenses showed only 40,000. 
The quality of an objective depends upon its chromatic 
and spherical corrections, flatness of field, and angular aper- 
