744 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 143. 



Fig. 144. 



Fig. 145. 



On further removing the objective the dark ring increases in size, 

 and when the upper part of the objective is in focus, we have (C") a 

 small white central disk, 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 pre- 

 venting their being confounded with the histological elements, enable 

 two general principles to be established, viz. — Bodies which are of 

 greater refractive power than the surrounding medium, have / a white 



centre which is sharper and smaller, 

 and a black ring which is larger 

 when the objective is withdrawn, 

 whilst those which are of less refrac- 

 tive power have a centre which is 

 whiter and smaller, and a black ring 

 which is broader and darker when 

 the objective is lowered. 



Monochromatic Light. — The same 

 phenomena are observed by yellow monochromatic light, except that 

 the diffraction fringes are more distinct, further apart, and in greater 

 numbers than with ordinary light. A fat-globule, indeed, seems to be 

 composed of a series of concentric layers like a grain of starch. 

 With blue light these fringes are also multiplied but are closer 

 together and finer, so that they are not so easily visible. Yellow 

 monochromatic light, therefore, constitutes a good means for deter- 

 mining whether the striae seen on an object are peculiar to it, or are 

 only diffraction lines. In the former case they are not exaggerated by 

 monochromatic light, but if, on the contrary, they are found to be 

 doubled, or quadrupled, with this light, we may be certain that they 

 are diffraction fringes. 



Figs. 144 and 145 show the appearance of air-bubbles in water, 

 when illuminated by yellow and blue monochromatic light. 



Atwood, M.— The Microscope in Metallurgy. [Supra, p. 735.] 



[Sep. Kepr. (from Newspapers) of papers read before the San Francisco 

 Microscopical Society.] 

 Beadle's (J.) Wire Clip for Mounting. 



[No description — " it is one of the best and simplest devices we have 

 seen."] 



Sci.-Gossip, 1882, p. 207. 



