ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



743 



only those rays which fall very close to the lower pole of the bubble 

 that will reach the eye, and the black marginal zone will therefore be 

 much larger. 



Fig. 142. 



I I 



This is shown in Fig. 142, 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 water (Fig. 142, 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 disk 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"), the central disk 

 becomes somewhat brighter, and is surrounded by a narrow black ring, 

 bordered within and without by diffraction circles. 



