. The microscope : an introduction to microscopic methods and to histology. Microscopes. ^ 106 INTERPRETATION 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. iffi 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 slid
. The microscope : an introduction to microscopic methods and to histology. Microscopes. ^ 106 INTERPRETATION 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. iffi 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,0). 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 â 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,âthe principal axis. F, principal focus. It is virtual and below for the air bubble; real and above for the oil globule. U20. Water or a mixture of water and gum arabic serving as a mounting medium ($ 140). 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 ape7'ture. 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
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