152 SELECTION AND USE 



ILLUMINATION OF TRANSPARENT OBJECTS. 



The different methods which have been devised for viewing 

 transparent objects are quite as numerous as those available for 

 opaque ones, and require quite as much tact and study. A 

 skilful worker, who thoroughly understands the points essential 

 to good, (3r rather to appropriate and efficient illumination, will 

 attain results wonderfully superior to those achieved by persons 

 ignorant of the subject, and this, too, although the latter may 

 be working with far superior instruments. This is seen every 

 season at our microscopical exhibitions and conversaziones, and 

 although the work done on these occasions is chiefly for show, 

 the same principle holds good in regard to work done in the 

 direction of study and investigation. 



Direct and Reflected Light. When the microscope 

 is so arranged that the light from a lamp or other self-luminous 

 body shall pass directly through the object and into the micro- 

 scope without being first reflected from the mirror, the illumin- 

 ation is said to be direct, in distinction from light which has 

 been first reflected from a mirror or other surface. Light from 

 a cloud or a white wall can scarcely be regarded as direct. 

 Direct light gives results which are appreciably different from 

 those produced by reflected light, since light always suffers a 

 change in character by reflection. These two kinds of illumin- 

 ation may be either axial or oblique, and in the case of both 

 reflected and direct light, if the source of light be very distant, 

 the rays will be sensibly parallel, but if the source of light be 

 very near, the rays will be divergent, and, consequently, under 

 such circumstances, the illumination must in part be more or 

 less oblique. 



Axial or Central Liglit. When the mirror, either 

 plane or concave, is placed directly in the axis of the micro- 

 scope, and reflects the light through the tube, the illumination 

 is said to be axial or central. The same term also applies to 

 direct light, when the direction in which the rays pass through 

 the object coincides with the optical axis of the instrument. 



