The Presidents Address. 
143 
assistance of the ordinary fine adjustment, without losing 
sight of the object under examination during the operation, an 
amount of convenience not to be credited but by those who 
have worked with both kinds of objectives. 
From the remarks that I have made upon aperture, it will 
be perceived that I differ materially from my predecessor in 
my appreciation of this element of utility in a microscopic 
object-glass. 
It is perfectly true, as stated by Dr. Carpenter, that with 
an objective of large aperture the perfection of its delinea- 
tion is more confined to one plane, and that consequently a 
general view of the object under consideration cannot be so 
well attained ; and secondly, that the manipulation is much 
more difiicult with an extended than with a contracted 
aperture. 
The very delicacy of one^s tools of course demands an extra 
amount of skill in their use ; but is it not possible to unite 
the advantage of a large aperture with the convenience of a 
moderate one in the same object-glass ? To this an affirma- 
tive reply may be given; for I have in my possession objec- 
tives made for me by Messrs. Smith and Beck, with a dia- 
phragm removeable at pleasure, so that for objects requiring 
aperture for their definition, that quality is attained with 
the most trifling amount of trouble to the manipulator, while 
for general use the aperture is contracted within moderate 
limits. Of course such a construction, to be eflPective, involves 
the proper correction for large angular aperture, in which case 
the lens will perform equally well when it is temporarily 
reduced. Messrs. Smith and Beck have fitted object-glasses 
in a similar manner for others besides myself, and 1 cannot 
but think it would be a great convenience if the higher powers 
especially were usually provided with this adaptation. 
A comparatively large aperture in the lower powers is use- 
ful in two ways, both in consequence of the extra amount of 
light transmitted, first, in examining opaque objects, which 
with such lenses scarcely require any aid from a condensing 
lens for illumination ; and secondly, the flood of light per- 
mitting the use of greater power in the eye-piece without 
too much reduction of brilliancy of the image, thus allowing, 
for a given amount of magnitude, the use of an objective of 
lower power than would otherwise be requisite, and conse- 
quently, what is sometimes of importance, a greater distance 
between the object and the lens. 
While on the subject of aperture, I cannot refrain from 
remarking, that I am convinced that in many cases some 
considerable error has been committed in the measurements 
