Objectives for the Compound Microscope. 411 
cover, their usual practice in this case being the reverse of their 
usual practice with regard to magnifying power. Now I have 
seen * eanal with an angle of 170° and upward at full cover 
correction, which did not exceed 140° at uncovered. is evi 
dent, therefore, that the maker should furnish with each glass 
h maximum and minimum angle, or the mieroscopist 
must measure for himself. 
After a full consideration of all the circumstances, I am dis- 
posed to think that the best interests of both makers and pur- 
chasers would be consulted if the present nomenclature were 
abandoned altogether, and objectives named instead by their pre- 
cise magnifying power without eye-piece at some selected dis- 
tance. It would be well if all the makers could be brought to 
agree on some fixed distance; but 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 num- 
ers. 
Thus we should have objectives without cover corrections 
named precisely 2, 8, 4, 5, 6, 7, 8, 9, and so on up to 100 or more, 
the number in icating the exact magnifying power, say at twelve 
and a half inches from micrometer to screen. Objectives with 
cover corrections would be named 80 to 40, 35 to 46, 75 to 89, 
125 to 140, &., the numbers representing the minimum and 
maximum magnifying powers at the selected distance. Objecs 
tives with wet and dry fronts would require a separate name for 
each; thus 78 to 95 dry, 98 to 180 wet, &e. ; 
_ By this plan the real magnifying power of the glass and its 
Imits of variation would be accurately stated, whereas at present 
even those makers who are most careful about their nomencla- 
ture do not hesitate to call a glass an }th provided its power at 
uncovered approximates that of a single lens of }th of an inch 
focus more nearly than it does a ith ora ;; th. Hence glasses 
Which differ materially in magnifying power at uncovered, re- 
ceive the same name, and the changes in power produced by the 
Cover correction, are invariably ignor Pee 
If the plan I here recommend be adopted, the precise distance 
™m micrometer to screen which may be chosen does not appear 
to me to be of very t importance. Many persons wou di 
Suppose 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- 
eat and stage micrometers, in the manner I shall presently 
escribe, if the distance be taken at 10 inches, whereas on most 
stands, with the help of the draw tube, 12} inches can be used 
