822 Use of various Microscojpes. [^S^lIi,BlT^ts^^ 
leaf had resolved the highest groups, a thing never before accom- 
plished with any instrument. This statement, however, is doubted 
by their learned countryman, Dr. Woodward. 
The test-plate of Nobert, dividing the inch into more than one 
hundred and twelve thousand parts, is generally adopted as a 
good test-object. But even here a very important consideration in 
forming a thorough and correct judgment exists, and is almost con- 
stantly overlooked ; I mean the difference in the aberration of the 
eyes of the observers. There is no doubt that different observers 
obtain different results from the same instrument ; of course a 
greater dissimilarity arises in the use of the same test-object with 
separate microscopes. All attempts to correct this personal aber- 
ration are still unreliable and unsatisfactory ; therefore the micro- 
scopic photographs which are brought to so admirable a degree of 
perfection, are, in fact, the surest test-objects now existing for the 
power of an instrument. 
Besides this personal difference there exists a very considerable 
one resulting from the continual use by each observer of one par- 
ticular instrument. In this connection I recall the striking fact, 
that as the celebrated microscope of Leeuwenhoek arrived at the 
Koyal Society in London after his death, no one was able to see the 
objects observed and described by him. An experienced observer 
will often see much better with his own imperfect instrument, to 
which he is accustomed, than another person would do with a far 
superior microscope. 
Doubtless the most important matter for microscopical science 
is the price for which the instrument can be obtained. The cheaper 
the instrument the larger the number of observers. In Europe, ten 
years ago, about two thousand large instruments were manufactured 
every year ; now the number is more than double. Surely for 
a physician, and for many other observers, an amplification of 
moderate size, from two hundred and fifty to three hundred dia- 
meters, is sufficient. Professor Ehrenberg, in Berlin — and I believe 
no living observer has made so much use of the microscope — 
uses almost constantly in his work an amplification of three hundred 
and fifty, and in some exceptional cases of seven hundred and fifty 
diameters. For histological purposes higher amplifications are 
necessary, but the physician and the naturalist will usually be con- 
tented with a good amplification of nearly three hundred diameters. 
Every possessor of a microscope wishes to test the power of his 
instrument, but it is not, and never has been, my purpose to pro- 
voke competition between American and European microscopes. 
Certainly every step toward the perfection of the microscope is 
important, but when the improvements are so minute that they 
cannot be used and seen easily and everywhere, they are, I think, 
jnore interesting to the artificer than to the operator. 
