186 Transactions of the Boyal Microscopical Societij. 
twenty-five years, must have been frequent witnesses of peculiarly 
evanescent forms, due to better or worse conditions of vision to which 
the instrument and the observer are occasionally subject. DiflPrac- 
tions of an unexpected character may reveal extraordinary improve- 
ment in defining power. If, for instance, a fine object-glass be used 
as a condenser, and the image of a gas flame lie in the field of view, 
perhaps at the edge of the flame, or at the edge of a piece of bright 
brasswork in the image, may be detected at once what had only been 
previously accomplished by long and patient labours. It can never 
be forgotten that all rays passing from a brilliant object up the 
microscope are not equally useful or equally potent in forming the 
correct image. The great art of perfecting definition from a given 
instrument often consists in destroying the useless and preserving 
the real working rays. General opinion seems gradually to have 
come round in favour of pin-hole stops for distinct definition ; 
the efiect of which is to limit the illuminating rays and prevent the 
object being drowned in excess of light. A pin-hole stop limits the 
illuminating pencil to perhaps ten degrees. But this stop is often 
in most dangerous proximity under the slide ; and the object may 
be pinclied between it and the nose of the objective, and cause 
destruction of both. An inch and a half objective * is generally of 
the same aperture (ten degrees), consequently the very same kind 
of illuminating pencil can be obtained from this as from the pin- 
hole without this source of annoyance : in addition to this, a Beck 
iris diaphragm attached below this kind of condenser gives every 
degree of fineness required in the illuminating pencil. Great 
advantage is also sometimes obtained by stopping off half the rays. 
In some cases of difficult beading, whether of scales or diatoms, 
I have found a U-shaped aperture highly advantageous. When a 
condenser is used with its axis obliquely inclined, some difficulty may 
be experienced in centering the lights. To obviate this, I have found 
a piece of transparently red paper gummed on to the front lens quite 
effectual, f The light shows itself through the same opaque paper, 
and informs the observer of the exact method of getting nearly the 
whole U aperture filled with the illuminating pencil. Very fine 
effects may sometimes be produced by a 4-inch opera-glass (used as 
a condenser), the eye-lens being removed : still finer, if such a glass 
be used to give parallel rays instead of the bull's-eye: the lamp 
flame being placed in its focus of course discharges a column of 
parallel rays upon the object. In proportion, however, as the rays 
become more nearly parallel, the light, if artificial, may be too much 
weakened. On this account a pencil of 10° or even 15° is pre- 
ferred, such as is given by a low-power objective. I have found in 
several investigations aplanatism in condensers quite as necessary 
* A pin-hole stop has about 10'^ of aperture. 
t Out of which a U-shaped aperture has been out. Blue paper is better, 
