Ch. II] EXPERIMENTS WITH A SUBSTAGE CONDENSER 6l 



Use a 4 mm. or higher objective and demonstrate the truth of the 

 above statements by looking at the same specimen, using the window 

 as a source of Hght and note the aperture of the objective which gives 

 the best effect. Then use one of the dayhght lanterns, and increase 

 the apertiire by opening the diaphragm until the best image is obtained. 

 Notice the detail sharply visible in the two cases. 



107a. The remarks of Wright, Principles of Microscopy, p. 219, are so 

 pertinent upon this point that they are here repeated : 



"The necessity for the regulation of the source of illumination will appear 

 when we consider the optical conditions which obtain where an extended 

 radiant field such as is furnished by the sky or a broad lamp flame is employed 

 as a source of light. There will be formed in such a case upon the stage of 

 the microscope by the focused condenser an image of the light source which 

 will extend beyond the limits of the field of view of any objective. From the 

 radiant points included within this illuminated area beams will pass into the 

 aperture of the objective. Those from the center of the field — always as- 

 suming that their numerical aperture does not exceed the numerical aperture 

 of the objective — will pass through the aperture unmutilated. It will be 

 different with respect to the beams which proceed from the periphery of the 

 field. These, taking the aperture obliquely, will, unless in the case where 

 their numerical aperture is much less than that of the objective, be cut down in 

 an unsymmetrical manner by the margin of the objective, exactly in the same 

 way as would be the case if transmitted through an elliptical, or, in the extreme 

 case, through a slit aperture. 



" It follows that while the radiant points in the center of "the field will be 

 represented in the image by circular antipoints whose dimensions will be 

 determined by the full numerical aperture of the objective, the radiant points 

 on the periphery of the field will be represented in the image by elliptical or 

 linear antipoints whose long axes will in each case be disposed radially to 

 the aperture, overlapping the antipoints in the center of the field in such a 

 manner as to fog the image." 



Experiments with the Condenser and Artificial Light 



§ 108. Kerosene lamp. — Use a kerosene lamp with a flat wick. 

 The wick should be from 3 to 5 cm. wide. For use, the flame is turned 

 up well, but not high enough to smoke. Put the lamp 15 to 20 cm. 

 from the microscope. Between the lamp and microscope should be 

 a dark screen suflSciently high and broad to shade the microscope stage 

 and to protect the eyes of the observer. The bottom of this screen 

 should be high enough from the table to admit the light (fig. 33) or 

 preferably it should have a hole from 5 to 10 cm. in diameter near its 

 lower edge to admit the light from the lamp (fig. 58). The dark screen 

 with the hole will serve also to hold the daylight glass (§ 92). 



