60 



is not sufficiently transparent to use this method ; the 

 object is then said to be opaque, and requires a dif- 

 ferent procedure. . We will say that it is desired to 

 examine an insect ; it may be attached to a slide, or, 

 what is better still, may be fastened in a stage for- 

 ceps (Fig. 16) as irmay then be turned and viewed 

 from all sides. The low power objective should 

 again be attached ; after having been focused, it will 

 be found that the light is insufficient to illuminatate 

 n^w^-^n-^ it. The mirror- 

 bar should now 

 be swung upon 

 Fig. 16. its axis around 



the stage to a point above it, so it will be at an angle 

 of about 45 degrees to its surface. If a lamp is used 

 and is in the same position as when used with trans- 

 mitted light, it is probable that the tube of the instru- 

 ment will obstruct the light, and it is then well to 

 move it toward the front. Using the concave mirror 

 adjust it so that the light will be concentrated upon 

 the object, by watching it directly, and then observe 

 through the tube. If it is not sufficiently illuminated 

 continue to adjust the mirror ; also vary its distance 

 from the object and swing the mirror-bar to a higher 

 or lower point. It often occurrs that, under the best 

 conditions, the need of better illumination is felt ; 

 in this case a bulls-eye condenser should be procured. 

 It will be found that this will become a useful and 

 perhaps necessary accessory in work outside of this. 

 Place it close to the instrument and set the bulls-eye 

 between the object and source of light, with \.\\e plane 



