USE OF THE COMPOUND MICROSCOPE 31 



bo somewhat higher above the inouutiiig lor spectacle 

 wearers than for normal eyes. 



If the eyepiece circle (of the objective) is larger than 

 the pupil of the observing eye, the used aperture of the 

 objective is cut dowai correspondingly, and there is a waste 

 of aperture, which is, however, the most costly thing about 

 a microscope. If the condenser eyepiece circle at the 

 eyepoint is equal to the pupil of the eye, the sHght move- 

 ments of the head, which are essential in looking through 

 the microscope, cause fluctuations in the light. Conse- 

 quently, it should be (and is usually) arranged in the 

 compound microscope that the aperture circles at the 

 eyepoint are smaller than the pupil of the eye. 



Fig. 10.- — Diagrams of an object seen through the microscope with a 3-milli- 

 meter diaphragm on the radiant, and concentric objectives of 10, 20, (iO, and 00 

 initial magnifying power. 



Illumination. — With microscopes of low power (as defined 

 above), the plane mirror may supply light of large enough 

 aperture to the object for transparent specimens, if a suffi- 

 ciently large source of light is used. Regarding the plane 

 mirror as a window through which the source of hght is seen 

 by an eye at the object, the aperture of the incident light 

 is proportional to the apparent breadth of that part of the 

 mirror which is seen to be filled with light from the source. 

 If the source is not large enough to fill the mirror, or if the 

 mirror is moved farther from the object, the aperture is 

 lessened. Hence, if the aperture is too small, the source of 

 light (perhaps a matt electric bulb) can be put closer; and 

 vice versa. The same is the case with the portion of sky 

 seen through a window. For these low powers, the maxi- 

 mum aperture for transparent objects is usually below 

 0.15. 



The concave mirror may be used, especially on low- 

 power microscopes, when there is a small source of light. If 



