ADJUSTMENT FOR FOCUS O 



latter will be employed almost exclusively in the work of this 

 volume, and its curvature is such that parallel rays, impinging 

 upon its surface, are focused about two inches from the mirror. 

 It will also be noticed that the bar, carrying the mirror -fork, may 

 be made to swing the mirror from side to side. The work whieh 

 we are about to undertake is of such a character as to require the 

 avoidance of oblique illumination. We must, therefore, keep our 

 mirror -bar strictly in the vertical position. If the mirror -bar 

 being vertical a line be drawn from the center of the face of the 

 mirror, through the opening (diaphragm) in the stage, passing on 

 through the objective, and so continued upward through the body 

 and the eye -piece, such a line would pass through the optical axis. 

 The center of the face of the mirror must be in this axis. If, 

 then, having gotten the mirror -bar properly fixed once for all, the 

 light from the adjacent right or left hand window impinges upon 

 the concave surface of the mirror, and the latter be properly 

 inclined, the rays will pass through the diaphragm in the stage, 

 and become focused a little above the same. The light rays will 

 afterward diverge, enter the objective, and finally reach the eye of 

 the observer. 



The field of view (as the area seen in the microscope is termed) 

 we will suppose to have been properly illuminated and by this we 

 mean that it presents us a clear, evenly lighted area. Turn all the 

 factors spoken of out of adjustment, and proceed to readjust. 

 Observe that, if the mirror be turned not swung slightly out of 

 proper position, one side of the field will appear dim or cloudy. 

 This must be corrected, and the student must practice until this 

 adjustment becomes easy of accomplishment. Then proceed to the 



ADJUSTMENT FOR FOCUS 



Swing the low -power objective into use, and rack the tube up 

 or down until it is about one -fourth of an inch from the stage. 

 Place a mounted object upon the stage (a stained section of 

 some organ say kidney will be preferable). Examine the field 

 through the eye -piece, and it will be found obscured by the stained 

 object, and perhaps a dim notion of figure may be made out. 

 Rack the body up carefully, watching the effect. The image 

 becomes more and more distinct until, at a certain point, the best 

 effect is secured. The object is in focus. 



Note carefully the distance between the object and the objective 



