Appendix A : The Microscope and Its Optical Principles 149 



Diaphragm. Opaque plates with openings of various sizes for regu- 

 lating the illumination of the object to be examined. The iris diaphragm 

 (Fig. 63) is the best type. It consists of a series of overlapping plates 



FIG. 63. Top View of a Substage Attachment with Condenser and Lower Iris 

 Diaphragm thrown out of Optical Axis. 



placed around a central opening the size of which may be varied by 

 means of a lever. Revolving diaphragms are commonly used on the 

 cheap grades of microscopes. They consist of round disks perforated 

 by openings of various sizes which may be rotated between the mirror 

 and the object. The nearer to the object the diaphragm is placed, the 

 better the intensity of the illumination can be regulated. Most of the 

 better class of microscopes are provided with two iris diaphragms, one 

 beneath the condenser to be employed when the latter is in use, the 

 other flush with the stage to be used only when the condenser is out. 

 If this second iris diaphragm is lacking, its place is taken by means of a 

 cap-diaphragm which may be fitted into the substage in the place of the 

 condenser. 



Dissecting Microscope. An instrument so constructed as to enable an 

 operator to carry on minute dissections under magnification. Ordinarily 

 they are simple microscopes mounted on a stand of some kind. The 

 best instruments (Fig. 64) are provided with well -corrected lenses, with 

 glass stage, mirror, black and white substage plate, and rests for the 

 hands. See also Figs. 65 and 66 for modified forms. 



Embryograph. A form of camera lucida for drawing at slight magni- 

 fication small objects, such as embryos. A camera lucida, attached to a 

 simple microscope is frequently used for this purpose. 



Eye-Point. The point above an ocular or lens at which the largest 

 number of rays from the instrument enter the eye. The largest field of 

 the microscope is visible from this point. 



