Microscopical Objectives and Oculars. 
19 
most convenient in use. If, however, the metric system were to 
come into general use, this distance would be changed to 250 milli- 
metres, with increased convenience, and with a scarcely appreciable 
difference in results. The sooner such a change is made the better, 
provided it is certain to come at all, and possibly it might be con- 
sidered only a fair concession to the convenience of the great 
number of Continental microscopists, and to the excellence of their 
metric system, to make this change without further delay. 
The propriety of measuring the image at this standard distance, 
when estimating the power of objectives or oculars is undisputed, 
and it would seem equally indisputable that the whole power of 
the compound microscope should be obtained in the same manner, 
were it not that the authorities have always differed in regard to 
the subject. When Hooke, Griffith, Hogg, and other eminent 
authorities have directed that the image should be measured at the 
distance of the object on the stage, and Lardner, Carpenter, and 
Suffolk, in common with most microscopists, measure the image 
10 inches from the eye wherever the object may be, it is useless to 
appeal to authorities. It would seem, however, that the former 
direction, to measure the image at the distance of the object, must 
be an advertency which could lead only to confusion. The writer 
has fully stated this question in a recent review,* and therefore 
omits further discussion of it here. 
A more difficult question is as to the point in the objective from 
which the measurement should be made. If the objective had an 
optical centre and we could find it, there would be no difficulty in 
the case. But the modern objective has no permanent optical 
centre, at least none that we can easily find and use, and unless 
some one can give us a better rule, we may be obliged to measure 
from the bottom of the whole system, or from (about) the centre of 
the lowest pair or set of lenses. Mr. Charles B. Cross! has pro- 
posed to evade this difficulty by measuring 10 inches between the 
conjugate foci used, without regard to the position of the objective — 
a plan which would be very eligible with high powers, but incon- 
venient if not inapplicable with low powers, since few compound 
microscopes have a body short enough to bring the conjugate foci 
within 10 inches of each other with very low objectives, and, if they 
did, the magnifying power, instead of being that generally used, 
would be greatly reduced or altogether suppressed. 
The very low-power objectives (say 4 and 5 inch) are usually 
mounted short in order to leave sufficient room between them and 
the stage, and their power, as ascertained by an arbitrary rule, 
would be greater than that at which they are usually worked, 
unless, in their ordinary use, the draw-tube were habitually raised 
enough to compensate for the shortness of their mounting. 
* The ‘ American Naturalist,’ June, 1871, p. 229. t Boston, 1870. 
