Ch. IX] 



DIFFRACTED LIGHT IN 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 95 % 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 



1.82s mm.. . 



1. 8s mm.. . 

 1.07s mm.. 

 LIS mm. . 

 1.4 mm.. . . 

 1.82s mm.. 

 1.82s mm.. 

 1.82s mm.. 

 1.825 mm.. 



Elevation of the Tube 



necessary to 



Restore the Focus 



Standard position 



No change of focus. 

 Tube raised 3I mm. 



3 mm. 



2 mm. 



tVtt mm. 

 . . T^Trmm. 



1%% mm. 



tVV 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 Ught is a necessary accompaniment. 

 The diffracted rays are shown by broken Unes 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 fight is of 

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

 generis) and not to be interpreted by ordinary geometric optics. 

 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 diffracted 

 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. 



