68 



VISION WITH THE COMPOUND MICROSCOPE 



fine details, only the outline remaining, and every delineation 

 of minute .structure disappearing just as if the microscope had sud- 

 denly lost its optical power (see fig. 53). 



This illustrates a case of the obliteration of structure by obstruct- 

 ing the passage of the diffraction-spectra to the eye-piece. 



The second experiment shows how the appearance of fine structure 

 may be created by manipulating the spectra. 



If a diaphragm such as that shown in fig. 54 is placed at the back 

 of the objective, so as to cut off' each alternate one of the upper row 

 of spectra in fig. 50, that row will obviously become identical with 

 the lower one, and if the theory holds good, we should find tin- image 

 of the upper lines identical with that of the lower. On replacing 

 the eye-piece we see that it is so : the upper set of lines are doubled 

 in number, a new line appearing in the centre of the space between 

 each of the old (upper) ones, and upper and lower sets having become 

 to all appearance identical (fig. 55). 



In the same way, if we stop off all but the outer spectra, as in fig. 

 56, the lines are apparently again doubled, ami are seen as in fig. 57. 



FIG. 57. 



A case of a [>parent creation of structure similar in principle to 

 the foregoing, though more striking, is afforded by a network of 

 s|iiares, such as fig. 5H, having sides parallel to the page, which gives 

 the spectra, shown in fig. 51), consisting of vertical rows for the 

 horizontal lines and horizontal rows for the vertical ones. But it 

 is readily seen that t\vo diagonal rows of spectra exist at right 



