26 MICROSCOPICAL TECHNIQUE. 



examined in a drop or two of either liquid, placed upon a slide 

 with a thin cover superposed after vigorously shaking the bottle 

 containing it. A drop of oil of turpentine coloured with magenta 

 or carmine, and a drop of water, may be placed on a slide together, 

 covered, and the cover moved about to cause them to mingle. 

 Globules of oil in water may also be studied in an emulsion pre- 

 pared by shaking the two together with a little powdered gum. 

 In an air-bubble in water, when the middle of it is focussed, the 

 centre of the image appears very bright, and it is surrounded by a 

 greyish zone, which in turn is encircled by a broad black ring 

 interrupted by one or more brighter ones. Outside the black ring 

 are diffraction circles, brighter than the field. On focussing down- 

 wards, the bright centre becomes smaller and brighter, and is 

 sharply divided from a very broad black ring which has bright dif- 

 fraction circles outside. Upward focussing, on the other hand, 

 causes the central portion to increase in size but become less 

 bright, whilst the now narrowed black ring is surrounded by nu- 

 merous diffraction circles. Air-bubbles in Canada balsam have 

 similar appearances in the different positions to those in water, but, 

 on account of the high refractive index of the balsam, the bright 

 central circle is smaller in each case. An oil-globule in water 

 shows the central disc brightest when the upper part of it is in 

 focus, and the broad black outer circle is not surrounded with 

 diffraction rings. Focussing down to the middle of the globule, 

 the disc becomes very large, but is much less bright, and the nar- 

 row black encircling ring is bordered by diffraction circles both 

 within and without. On lowering the objective yet further the 

 bottom of the globule appears as a grey disc, somewhat darker 

 than the field, and separated from it by a darker ring. 



HandsomeCell-material.— Celluloid.— A writer in the National 

 Druggist (xxiii., 1883, pp. 28 and 42) gives the following descrip- 

 tion of a method for using celluloid as a cell material : — "Having 

 at hand a few sheets of celluloid imitation ivory of high grade, the 

 idea struck him that this was the very thing, and a little experimen- 

 tation proved that the idea was correct. The sheets were not thick 

 enough to make a cell of the required depth, but this was no draw- 

 back, since celluloid is easily cemented. With sharp punches of 

 proper size he cut out a sufficient number of rings from the sheets 



