140 UNIFORMITY OF NOMENCLATURE IN OBJECTIVES. 
The smaller numbers were evidently too small, and the last 
inches, seems to be permanently accepted as most correct theo 
cally and most convenient in use. If, however, the metric syst 
were to come into general use, this distance would be change 
two hundred and fifty millimetres with increased convenience 
with a scarcely appreciable difference in results. The sooner 
a change is made the better, provided it is certain to come at all 
and possibly it might be considered only a fair concession to 
convenience of the great number of continental microscopists 
to the excellence of their metric system, to make this a 
out further delay. 
The propriety of measuring the image at this standard dist 
when estimating the power of objectives or oculars is undispt 
and it would seem equally undisputable that the whole powe 
the compound microscope should be obtained in the same mal 
were it not that the authorities have always differed in regard to th 
subject. When Hooke, Griffith, Hogg, and other eminent aw 
ities have directed that the image should be measured at the 
tance of the object on the stage, and Lardner, Carpenter and 
folk, in common with most microscopists, measure the image | 
inches from the eye wherever the object may be, it is useless to 
peal to authorities. It would seem, however, that the former cuit 
tion, to measure the image at the distance of the object, must 
an adverteney which could lead only to confusion. The writer 
fully stated this question in a recent review,* and theren 
further discussion of it here. 
A more difficult question is as to the point in the object 
which the measurement should be made. If the objective 
optical centre and we could find it, there would be no diflic u 
the case. But the modern objective has no permanent opue 
tre, at least none that we can easily find and use, and unless 
one can give us a better rule, we may be obliged to meas™ 
the bottom of the whole system, or from (about) the cent 
lowest pair or set of lenses. Mr. Charles R. Cross} has pr 
to evade this difficulty by measuring ten inches between 
_ jugate foci used, without regard to the position of the obje 
plan which would be very eligible with high powers, but 
venient if not inapplicable with low powers, since few co 
*The American Naturalist, June, 1871, p- 229- 
t Boston, 1870 
