86 INTERPRETATION OF APPEARANCES. [CH. III. 



line as both differ greatly in refractive power from the medium which 

 surrounds them, the oil being more refractive than the mucilage and 

 the air less. (Figs. 53-55). 



Plaee a fragment of a cover-glass on a clean slide, and cover it (see 

 under mounting). The outline will be very distinct with the unaided 

 eye. Use it as object and employ the 16 mm. (73 in.) objective and 

 high ocular. Light with central light. The fragment will be outlined 

 by a dark band. Put a drop of water at the edge of the cover-glass. 

 It will run in and immerse the fragment. The outline will remain dis- 

 tinct, but the dark band will be somewhat narrower. Remove the cover- 

 glass, wipe it dry, and wipe the fragment and slide dry also. Put a 

 drop of 50% glycerin on the middle of the slide and mount the fragment 

 of cover-glass in that. The dark contour will be much narrower than 

 before. 



Draw a solid glass rod out to a fine thread. Mount one piece in air, 

 and the other in 50% glycerin. Put a cover-glass on each. Employ 

 the same optical arrangement as before. Examine the one in air first. 

 There will be seen a narrow, bright band, with a wide, dark band on 

 each side. 



The one in glycerin will show a much wider bright central band, with 

 the dark borders correspondingly narrow (Fig. 89 b). The dark contour 

 depends also on the numerical aperture of the objective — being wider 

 with low apertures. This can be readily understood when it is remem- 

 bered that the greater the aperture the more oblique the rays of light 

 that can be received, and the dark band simply represents an area in 

 which the rays are so greatly bent or refracted (Figs. 53, 55) that the}' 

 cannot enter the objective and contribute to the formation of the image ; 

 the edges are dark simply because no light from them reaches the ob- 

 server. 



Frc 89. Solid glass rod showing the ap- 

 pearance when viewed with transmitted, 

 central light, and with an objective of medi- 

 um aperture. 



a. Mounted in air. b. Mounted in 50 per cent, glycerin. 



If the glass rod or any other object were mounted in a medium of the 

 same color and refractive power, it could not be distinguished from the 

 medium.* 



* Some of the rods have air bubbles in them, and then there results a capillary 

 tube when they are drawn out. It is well to draw out a glass tube into a fine thread 

 and examine it as described. The central cavity makes the experiment much more 

 complex. 



