bull’s-eye condenser. 
59 
stage by a bayonet joint. The dia- 
phragm having a variety of open- 
ings of different form and size, turns 
upon a pivot so situated that each , 
opening is successively brought into I 
the axis of the microscope. The® 
space between the largest and small- 
est openings is greater than between 
any other two, and is designed to exclude all transmitted light 
and give a dark background when viewing opaque objects. 
A bent spring is attached to the fixed part, and rubs against 
the edge of the movable plate, which is provided with notches, 
so arranged that when either of the holes is brought into its 
proper position the end of the spring drops into the notch. 
In the plain form of diaphragm all the openings are circular? 
but in the more expensive kinds, openings of a variety of 
forms are employed, some excluding the central rays, others are 
crescent shaped, or semi-circular, admitting only light from one 
side. 
When the eye becomes fatigued by too strong light, or by 
the intense glare or yellow rays of artificial light, relief is 
afforded by inserting in the body of the diaphragm a piece of 
gray, neutral tint, or light blue glass, by which the light can 
be modified to any extent required. 
80. Bull’§-Eye Condenser. This is a large plano-convex 
lens, mounted on a brass stand and pillar. The lens is attached 
by a cradle joint to a revolving arm, which is supported by a 
sliding tube, so cut as to act as a spring and retain the arm and 
lens steady at any elevation and in any position. For illumina- 
ting opaque objects, the bull’s-eye is so adjusted above the 
stage, with its fiat side towards the object, as to bring the light 
to a focus upon the object on the stage. In using artificial light, 
the large bull’s-eye is to be placed with its plane surface 
towards the light, and so adjusted that the beam of light 
transmitted shall be about the size of the mirror, or of the 
smaller bull’s-eye, when the rays will be nearly parallel, or 
slightly converging. The mirror, or smaller bull’s-eye, may 
then be used to bring the light to a focus upon the obj ect. If the 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
