17 MICROSCOPE AND ACCESSORIES. 



LIGHTING AND FOCUSING : EXPERIMENTS. 



§ 46. Lighting with a Mirror. — Put a mounted fly's wing (see Ch . 

 V, under mounting), under the microscope, put the ^ in. (18 mm.) or 

 other low objective in position, also a low ocular. With the coarse ad- 

 justment (Fig. 10), lower the body of the microscope within about 1 

 cm. of the object. Use an opening in the diaphragm about as large as . 

 the front lens of the objective ; then with the plane mirror try to reflect 

 light up through the diaphragm upon the object. One can tell when 

 the field (§ 33) is illuminated, by looking at the object on the stage, but 

 more satisfactorily by looking into the microscope. It sometimes re- 

 quires considerable manipulation to light the field well. After using 

 the plane side of the mirror turn the concave side into position and light 

 the field with it. A.s the concave mirror condenses the light, the field 

 will look brighter with it than with the plane mirror. Is it especially 

 desirable to remember that the excellence of lighting depends in part 

 on the position of the diaphragm (§ 45). If the greatest illumination 

 is to be obtained from the concave mirror, its position must be such that 

 its focus will be at the level of" the object. This distance can be very 

 easily determined b}^ finding the focal point of the mirror in full sun- 

 light. 



§ 47. Use of the Plane and of the Concave Mirror. — The mirror 

 should be freely movable, and have a plane and a concave face. The 

 concave face is used when a large amount of light is needed, the plane 

 face when a moderate amount is needed or when it is necessary to have 

 parallel rays or to know the direction of the rays. 



§ 48. Focusing with Low Objectives. — Place a mounted fly's wing 

 under the microscope ; put the three fourths (18 mm.) objective in po- 

 sition, and also the lowest ocular. Select the proper opening in the 

 diaphragm and light the object well with transmitted light (§41, 48). 



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

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

 coarse adjustment lower the body (Fig. 10), until the objective is within 

 about half a cm. of the object. Then look into the microscope and 

 slowly elevate the body with the coarse adjustment. The image will 

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

 still higher. If the body is raised too high the image will become in- 

 distinct, and finally disappear. It will again appear if the body 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. Put a high oc- 

 ular in place of the low one (§ 27, 29, 31). If the oculars are not par- 



