STUDIES IN INTERPRETATION 



429 



bubbles of air in water or Canada balsam. This comparison may 

 be very readily made by shaking up some oil with water to which 

 a little gum has been added, so as to form an emulsion, or by 

 simply placing a drop of oil of turpentine (coloured with magenta 

 or carmine) and a drop of water together upon a slide, laying a thin 

 glass cover over them, and then moving the cover backwards and 

 forwards several times on the slide. Equally instructive are the 

 appearances of an air-bubble in water and Canada balsam. 



The figures which illustrate the appearance at various points 





- -jr^'lJ - "^ -^ 



(3 



FIG. 366. Air-bubbles in (1) water ; (2) Canada balsam; (3) fat-globule8 in water. 



of the focus of an air-bubble in water and Canada balsam, and of a 

 frit-globule in water, may be thus illustrated, viz. a diaphragm of 

 about f of a mm. being placed at a distance of 5 mm. beneath the 

 stage, and the concave mirror exactly centred. 



Air-babbles in water. No. 1 (fig. 366) represents the different 

 appearances of an air-bubble in water. On focussing the objective 

 to the middle of the bubble (B), the centre of the image is seen to be 

 very bright brighter than the rest of the field. It is surrounded by 

 a greyish zone, and a somewhat broad black ring interrupted by one 



