94 
NOTES AND MEMORANDA. 
objectives. Practically 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 must have an optical centre as truly as a single lens ; but 
the difficulty is to find its place among the lenses first, and then to 
fin d 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 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 ? He says the least, 
because it is the fixed point, always to be found ; because it repre- 
sents 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 test of what their real focus should 
prove to be. Angular aperture varies also with the change of cover 
adjustment, and the practice is to give that at its maximum only ; 
Tolies, now, however, gives and marks sometimes both maximum and 
minimum. Dr. Ward suggests that it should be given at the same 
point as the name. It is usually at the minimum there ; but it may 
be anywhere else ; Tolies can make it the maximum at open point. 
The question has been raised in regard to objectives with two front 
lenses, interchangeable, of different powers, as to what they should be 
called. Obviously they are practically two instruments, and should 
have the names proper to each front. This may be illustrated thus : 
Tolies will make an objective dry T \j-, and to that put another front 
^jj. immersion, would it not be preposterous to call the performance of 
the immersion front that of a -J^th ? His practice is to give two 
names ; and as the originator of the plan of two fronts he may have 
the right to fix the rule of nomenclature. 
The Biological Secretaryship of the Royal Society. — In this we 
of course are interested as well as the macro-biologists. The loss of 
Dr. Sharpey will be a very great one ; for with his great and extended 
knowledge he possessed a most kindly and painstaking manner, which 
we ourselves have frequently experienced. We trust that his post 
may be filled by an equally worthy person, and assuredly we know of 
none so highly qualified for the office as Professor Huxley, whom we 
hope to see soon in Dr. Sharpey’s place. 
