56 ARTIFICIAL LIGHT FOR THE MICROSCOPE [CH. II 



100 mm. equivalent focus, the entire field might not be lighted with 

 so small a disc of daylight glass. 



For object, use a fly's wing or any good, well-stained specimen. 

 It would be interesting to sit near a window, and to turn the mirror 

 in such a way as to bring in daylight a part of the time. In this way 

 one can get a good idea of the real similarity of the artificial and of 

 the natural daylight. If one also had an electric lamp without 

 any light filter one could pass in order from real daylight, through 

 the artificial daylight and then on to the unmodified artificial light. 

 Without seeing these in comparison, one is hardly able to appreciate 

 the likeness between the natural and artificial daylight and the great 

 unlikeness of unfiltered electric light and artificial daylight. 



CENTRAL AND OBLIQUE LIGHT WITH A MIRROR 

 98. Axial or central light (86). Remove the condenser or 

 any diaphragm from the substage, then place a preparation containing 

 minute air bubbles under the microscope. The preparation may be 

 easily made by beating a drop of mucilage on the slide and covering 

 it (see Chs. IX-X). Use a 4 mm. objective and a 4x or 5x ocular. 

 Focus the microscope and select a very small bubble, one whose 

 image appears about i mm. in diameter, then arrange the plane 

 mirror so that the light spot in the bubble appears exactly in the 

 center. Without changing the position of the mirror in the least, 

 replace the air bubble preparation by one of Pleurosigma angulatum 

 or some other finely marked diatom. Study the appearance very 

 carefully. 



99. Oblique light (87). Swing the mirror far to one side 

 so that the rays reaching the object may be very oblique to the optic 

 axis of the microscope. Study carefully the appearance of the diatom 

 with the oblique light. Compare the appearance with that where 

 central light is used. The effect of oblique light is not so striking with 

 histological preparations as with diatoms. 



It should be especially noted in 98-99, that one cannot determine 

 the exact direction of the rays by the position of the mirror. This is 

 especially true for axial light (98). To be certain the light is axial 

 some such test as that given in 195 should be applied. 



