22 THE MICROSCOPE. 



alone, only the contour of the object is seen, the addi- 

 tion of at least one diffraction spectrum being essential 

 to the visibility of the structure. 



When by a diaphragm placed at the back of the objec- 

 tive, as in fig. 6, we cover up all the diffraction spectra of 

 fig. 5,and allow only the central rays to reach the image, 

 the object will appear to be wholly deprived of fine 



FIG. 6. 



FIG. 6a. 



details, the outline alone will remain, and every deline- 

 ation of minute structure will disappear, just as if the 

 microscope had suddenly lost its optical power, as in 

 fig. 60. 



This experiment illustrates a case of the obliteration 

 of structure by obstructing the passage of t^e diffrac- 

 tion spectra to the eye-piece. The next experiment 

 shows how the appearance of fine structure may bo 

 created by manipulating the spectra. 



Fio. 7. 



FIG. 7o. 



When a diaphragm such as that shown in fig. 7 is 

 placed at the back of the objective, so as to cut off 

 each alternate one of the upper row of spectra in 

 fig. 5, that row will obviously become identical with 

 the lower one, and if the theory holds good, we 



