io6 



INTERPRE TA TION OF APPEARANCES [ CH. Ill 



(see under mounting). The outline will be distinct with the un- 

 aided eye. Use it as object and employ the 16 mm. ( 2 3 in.) objec- 

 tive 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 distinct, but the dark band will be somewhat 

 narrower. Remove the cover-glass, wipe it dry, and wipe the frag- 

 ment 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. 



FIG. 99. Section of an air bub- 

 ble and an oil globule in water (H 2 O). 

 The air bubble although spherical in 

 form gives only a virtual focus, indi- 

 cated by the dotted lines below the bub- 

 ble. As it is surrounded by a denser 

 medium it acts like a concave lens in 

 air (Fig. 10} . The focus of the oil 

 globule is real as it is denser than the 

 surrounding medium. Axis, the 

 principal axis. F, principal focus. It 



is virtual and below for the air bubble ; real and above for the oil globule. 



H^O. Water or a mixture of water and gum arabic serving as a mounting 



medium (\ 



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 (Fig. 100, a;. 



FIG. 100. Solid glass rod showing 

 the appearance when viewed with 

 transmitted, central light, and with an 

 objective of medium aperture. 

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



The one in glycerin will show a much wider bright central 

 band, with the dark borders correspondingly narrow (Fig. 100, 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 remembered that the greater the aperture the 

 more oblique the rays of light that can be received, and the dark 



