430 MANIPULATION AND PRESERVATION OF THE MICROSCOPE 



or more brighter circles. Round the black ring are again one or 

 more concentric circles (of diffraction), brighter than the field. 



On focussing to the bottom of the bubble (A) the central white 

 circle diminishes and becomes brighter ; its margin is sharper, and 

 it is surrounded by a very broad black ring, which has on its 

 periphery one or more diffraction circles. 



When the objective is focussed to the upper surface of the 

 bubble (C) the central circle increases in size, and is surrounded by 

 a greater or less number of rings of various shades of grey, around 

 which is again found a black ring, but narrower than those in the 

 previous positions of the objective (A and B). The outer circles of 

 diffraction are also much more numerous. 



Air-bubbles in Canada balsam. Canada balsam being of a 

 higher refractive index than water, the limiting angle, instead of 

 being 48 35', is 41 only, so that the rays which are incident much 

 less obliquely on the surface of separation undergo total reflexion, 

 and it will be only those rays which face very close to the lower 

 pole of the bubble that will reach the eye, and the black marginal 

 zone will therefore be much larger. This is shown in fig. 366, No. 2. 



When the objective is focussed to the bottom of the bubble 

 (A'), we have a small central circle, brighter than the rest of the 

 field, all the rest of the bubble being black, with the exception of 

 some peripheral diffraction rings. On focussing to the centre (B') 

 or upper part (C ; ) of the bubble, we have substantially the same 

 appearances as in B and C, with the exception of the smaller size 

 of the central circle. 



Fat-globules in ivater (fig. 366, No. 3). These illustrate the 

 case of a highly refracting body in a medium of less refractive power. 



When the objective is adjusted to the bottom of the globule A", 

 it appears as a grey disc a little darker than the field, and separated 

 from the rest of the field by a darkish ring. 



Focussing to the middle of the bubble (B r/ ), the central disc 

 becomes somewhat brighter, and is surrounded by a narrow black 

 ring, bordered within and without by diffraction circles. 



On further removing the objective the dark ring increases in 

 size, and when the upper part of the bubble is in focus, we have 

 (C r/ ) a small white central disc, brighter than the rest of the field, 

 and sharply limited by a broad, dark ring which is blacker towards 

 the centre. 



These appearances are the converse of those presented by the 

 air-bubble. That, as we saw, has a black ring and a white centre, 

 which are the sharper as the objective is approached to the lower 

 pole of the bubble. The fat-globule has, however, a dark ring 

 which is the broader, and a centre which is the sharper, according 

 as the objective is brought nearer to the upper pole. 



These considerations, apart from their enabling us to distinguish 

 between air-bubbles and fat-globules, and preventing their being 

 confounded with the histological elements, enable two general 

 principles to be established, viz. bodies which are of greater re- 

 fractive power than the surrounding medium have a white centre 

 which is sharper and smaller, and a black ring which is larger when 



