ADJUSTMENT FOE FOCUS. 5 



that the bar, carrying the mirror-fork, may be made to swing the 

 mirror from side to side. The work which we are about to under- 

 take is of such a character as to require the avoidance of oblique il- 

 lumination. We must, therefore, keep our mirror-bar strictly in the 

 vertical position. If the mirror-bar being vertical a line be drawn 

 from the centre of the face of the mirror, through the opening (dia- 

 phragm) in the stage, passing on through the objective, and so con- 

 tinued upward through the body and the eye-piece, such a line would 

 pass through the optical axis. The centre 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 focussed 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 I mean 

 that it presents as 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 practise until this adjustment becomes easy of 

 accomplishment. Then proceed to the 



ADJUSTMENT FOE FOCUS. 



Observe that the largest opening in the stage diaphragm is in the 

 optical axis. Swing the low-power objective into use, and rack the 

 tube up or down until it is about one 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. Eack the body 

 down 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 

 (with the Hartnack No. 2 this will be about seven-eighths of an inch), 

 and hereafter you will be able to focus more quickly. 



Having observed the details of structure as shown with the inch 

 objective, swing the high power into use. Rack the tube down, until 

 the objective is about one-eighth of an inch from the glass covering 



