CH. II] FOCUSING THE MICROSCOPE 37 



FOCUSING EXPERIMENTS 



70. Focusing low objectives. Place a mounted fly's wing 

 under the microscope; put the 16 mm. objective and the 4x or 5x 

 ocular in position. Select the proper opening in the diaphragm and 

 light the object well with transmitted light ( 85). 



Hold the head at about the level of the stage, look toward the 

 window, and between the object and the front of the objective; with 

 the coarse adjustment lower the tube until the objective is within 

 about half a centimeter of the object. Then look into the microscope 

 and slowly elevate the tube with the coarse adjustment. The image 

 will appear dimly at first, but will become very distinct by raising the 

 tube still higher. If the tube is raised too high the image will become 

 indistinct, and finally disappear. It will again appear if the tube is 

 lowered the proper distance. 



When the microscope is well focused try both the concave and the 

 plane mirrors in various positions and note the effect. 



Pull out the draw- tube 4 to 6 cm., thus lengthening the body of 

 the microscope; it will be found necessary to lower the tube of the 

 microscope somewhat (for reason, see fig. 83). 



71. Pushing in the draw-tube. To push in the draw-tube, 

 grasp the large milled ring of the ocular with one hand, and the milled 

 head of the coarse adjustment with the other, and gradually push 

 the draw-tube into the tube. If this were done without these pre- 

 cautions the objective might be forced against the object and the 

 ocular thrown out by the compressed air. 



72. Focusing with high objectives. Employ the same object 

 as before, elevate the tube of the microscope and, if no revolving nose- 

 piece is present, remove the 16 mm. objective as indicated. Put a 

 4 mm. or higher objective in place, and use a 4x or 5x ocular. 



Light well, and employ the proper opening in the diaphragm, etc. 

 ( 89). Look between the front of the objective .and the object as 

 before ( 70), and lower the tube with the coarse adjustment till the 

 objective almost touches the cover-glass over the object. Look into 

 the microscope, and with the coarse adjustment, raise the tube very 

 slowly until the image begins to appear, then turn the milled head of 



