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MICROSCOPY. 377 
cally it is next to impossible to apply the rule with mathematical 
exactness. Mr. Cross says, ‘‘ because the compound objective has 
no optical centre ;” but this is an error; several lenses combined 
mast have an optical centre as truly as a single lens; but the dif- 
fieulty is to find its place among the lenses first, and then to find 
where that place is on the outside of the tube. But for high 
power objectives it is so near the point between the front and 
middle combinations, that if that point is taken, there can be 
W hardly an appreciable difference from the truth in the result. The 
optical centre of the ocular is the diaphragm, if that is in the 
place where it should be. 
But in the modern objective the optical centre is a movable 
point by the collar for cover adjustment. The nearer the lenses 
are brought together the more the combination magnifies. Shall 
the objective be named at least power, or some other. I say the 
least, because it is a fixed point, always to be found; because it 
represents the least the instrument can do, and the owner knows 
that the power increases from that point; and because it seems to 
have been adopted in theory by most of the best makers, as but few 
of their medium powers have ever been found “over named.” If 
they did not proceed on that principle, their objectives were still 
farther from what they were named than they have been found to 
be by that rule. It would certainly be desirable that makers 
should give the minimum as well as the maximum of focus for 
each objective. The practice of most makers seems to have been 
-to make certain lenses by rule of thumb, to be called e. g. a fourth, 
Put them together, give them the name, and sell them without any ` 
lest of what their real focus should prove to be. 
Angular aperture varies also with the change of cover adjust- 
t, and the practice is to give that at its maximum only ; 
Tolles, now, however, gives and marks sometimes both maximum 
inimum. . Ward suggests that it should be given at the 
same point as the name. It is usually at the minimum there ; but 
may be anywhere else; Tolles can make it the maximum at 
Pen point. i. 
r ? The question has been raised in regard to objectives with two 
9 enses, interchangeable, of different powers, as to what they 
” m ld be called. Obviousl y they are practically two instruments, 
_ i should have the names proper to each front. This may be 
‘ ted thus: Tolles. will make an objective dry yo, and to 
