Objectives for the Compou nd Microscope. 
71 
at uncovered. It is evident, therefore, that the maker should furnish 
with each glass both the maximum and minimum angle, or the 
microscopist must measure for himself. 
After a full consideration of all the circumstances, I am disposed 
to think that the best interests of both makers and purchasers 
would be consulted if the present nomenclature were abandoned 
altogether, and objectives named instead by their precise magnifying 
power without eye-piece at some selected distance. It would he 
well if all the makers could be brought to agree on some fixed 
distance ; hut until we obtain this happy uniformity, which perhaps 
is not to be anticipated, it is only necessary for each maker to state 
the distance he selects. By this plan objectives without correction 
for cover would be named by one number, objectives with correction 
by two, and those with two or more fronts or backs by two or more 
pairs of numbers. 
Thus we should have objectives without cover corrections named 
precisely 2, 3, 4, 5, 6, 7, 8, 9, and so on up to 100 or more, the 
number indicating the exact magnifying power, say at 124 inches 
from micrometer to screen. Objectives with cover corrections 
would be named 30 to 40, 35 to 46, 75 to 89, 125 to 140, Ac., 
the numbers representing the minimum and maximum magnify- 
ing powers at the selected distance. Objectives with wet and dry 
fronts would require a separate name for each; thus 78 to 95 dry, 
98 to 130 wet, &c. 
By this plan the real magnifying power of the glass and its 
limits of variation would be accurately stated, whereas at present 
even those makers who are most careful about their nomenclature 
do not hesitate to call a glass an §4h provided its power at un- 
covered approximates that of a single lens of g-th of an inch focus 
more nearly than it does a |th or a T Vtli. Hence glasses which 
differ materially in magnifying power at uncovered, receive the 
same name, and the changes in power produced by the cover cor- 
rection are invariably ignored. 
If the plan I here recommend be adopted, the precise distance 
from micrometer to screen which may be chosen does not appear to 
me to be of very great importance. Many persons would, I sup- 
pose, prefer 10 inches to 12^. I have selected the latter number 
because many of our larger stands have tubes too long to permit 
the convenient measurement of low powers with eye-piece and stage 
micrometers, in the manner I shall presently describe, if the distance 
he taken at 10 inches, whereas on most stands, with the help of the 
draw-tube, 12 \ inches can be used conveniently with all powers 
from the 3-inch upwards. I do not, however, insist on any parti- 
cular distance, but only that the distance selected shall be stated 
in each case until some uniform plan shall be generally agreed 
upon. 
