INTRODUCTION 23 



is an opening for the passage of light by which an object is 

 to be illuminated. It is provided with clips for firmly 

 holding the object in position during its study. Situated 

 just under the lower side of the stage is a mechanism, (d) 

 the diaphragm, for regulating the amount of light admitted 

 to the object. Note that these diaphragms may be of 

 different types, mere disks to be inserted in the stage, or a 

 circular disk to be rotated, or a shutter-like device known 

 as the iris diaphragm, (e) Attached to a movable arm 

 under the stage is the mirror by which light is reflected through 

 the object and lens to the eye of the observer. Note that 

 it is double, having on one side a plane, and on the other 

 a concave surface; the latter serves to concentrate more 

 light upon the object, and should be used chiefly with the 

 high power lenses. 



Above the stage is (/) the tube supported on the arm of 

 the microscope. The tube is a means of attaching the 

 optical parts, i. e., the lenses, of which there are several; 

 those at the upper end being called the oculars or eye-pieces 

 since they relate directly to the eye of the observer; those 

 attached to the lower end known as the objectives, since 

 they relate to the object under observation. The wheel- 

 like parts working on the side of the arm, and a similar 

 smaller one at the top or side, have to do with focusing to 

 be explained later. 



2. Adjustments. These refer to the matter of so relating 

 the mirror, the object to be studied, the lenses employed, 

 the amount of light admitted, that clear and distinct images 

 are afforded. First in the process is that of light, and 

 practice will be required in order to learn its importance. 

 This will involve testing the effects of both plane and con- 

 cave mirrors, the use of the diaphragm in regulating the 

 amount of light, and finally that adjustment of the lenses 



