Ch. IX] 



DIFFRACTED LIGHT IX MICROSCOPY 



301 



Tester and Liquid in the Concavity 



No tester used 



Whole thickness of the tester at one end, not 



over the cavity 



Tester with water 



Tester with 05 % alcohol 



Tester with kerosene 



Tester with Gundlach Opt. Co.'s hom. liquid 



Bausch & Lomb Opt. Co.'s hom. liquid 



Leitz' hom. liquid 



Zeiss' hom. liquid 



Size of the 

 Field 



Elevation of the Tube 

 no essary i" 

 1 Mr.- the InCUS 



r.825 mm.. . . Standard position 

 1.85 mm No < hange of fo< us. 



1.075 mm. 

 1. 15 mm. . 

 1.4 mm.. . . 

 1.825 mm.. 

 1.825 mm.. 

 1.825 mm.. 

 1.825 mm.. 



Tube ra 



sed v} mm. 

 3 mm. 

 2 mm. 



mm. 



mm. 



mm. 



mm. 



when the object must be viewed with direct light from some outside 

 source. 



In traversing small orifices or slits and objects with minute details 

 the spreading out of diffracted light is a necessary accompaniment. 

 The diffracted rays are shown by broken lines in the accompanying 

 figures from Wright (fig. 178-179). As seen from these there may be 

 two systems of diffracted rays, one from the object and another from 

 the border of the objective, and these two systems of diffracted rays 

 act differently. 



The role played by the diffracted light has been variously inter- 

 preted by opticians. By Abbe and his adherents diffracted light is of 

 supreme importance, and microscopic vision is a thing by itself (sui 

 generis) and not to be interpreted by ordinary geometric optic-. 

 Certain very striking experiments have been devised to show the accu- 

 racy of this hypothesis, but as pointed out by many, the ordinary use 

 of the microscope never involves the conditions realized in those 

 experiments. 



While the supreme importance ascribed by some to the diffrai ted 

 light may not be accepted, no one will deny the presence of diffraction 

 phenomena in miscroscopic vision. If, furthermore, the diffracted rays 

 are brought by the microscope to the final focus with the undiffracted 

 light passing from the object through the microscope, the image will 

 be conceivably more perfect than as if the diffracted rays produce 

 secondary images, or mere blur. 



