200 



THEORY OF MICROSCOPIC OBSERVATION. 



nearer to the centre in blue light than in red ; and, consequently, 

 when viewed with white light they appear encircled on the inside 

 with a blue fringe and on the outside with a red one. 



The bright rings, and at the same time the fringes of colour, 

 appear very distinct at the sides facing the adjacent air-bubbles. 

 The faint half-light, which gives a grey tone to the marginal 

 shadow, becomes here intensified to a band of light, which towards 

 the interior admits of the recognition of distinctly indicated lines 

 corresponding to the rings, while on the other hand a homogeneous 

 glimmer is perceived in the outward direction. How such a re- 

 inforcement of the luminous effect is brought about by the action of 



FIG. 109. 



one air-bubble upon another one, may be demonstrated in a simple 

 manner by tracing the rays of the emergent cone of light back- 

 wards, and, at the points where they are reflected, allowing them to 

 pass into water by refraction, whereby they are partially reflected 

 downwards towards the diaphragm by the adjacent air-bubble. In 

 this mariner, for instance, the rays a c and b p (Fig. 109), which 

 converge to the point o in the second ring of the air-bubble A 

 and after a two-fold refraction proceed in the directions df and e y, 

 reach h and i by reflexion on the air-bubble B. If a c and b p are 

 the marginal rays of a cone of light incident from above, then / Ji 

 and g i are the marginal rays of the corresponding emergent cone. 



