THEORY OF MICROSCOPICAL VISION. 



21 



decreasing in intensity as they are further removed 

 from the direction of the incident beam transmitted 

 through the structure, the interference of the primary 

 waves giving a number of successive maxima of light 

 with dark interspaces. 



When a diaphragm opening is interposed between the 

 mirror, and a plate of ruled lines placed upon the stage 

 such as fig. 5, the appearance shown in fig. 5a, will be 

 observed at the back of the objective on removing the 

 eye-piece and looking down the tube of the microscope. 

 The centre circles are the images of the diaphragm 

 opening produced by the direct rays, while those on 

 the other side (always at right angles to the direction 

 of the lines) are the diffraction images produced by 

 the rays which are bent off from the incident pencil. 



Fro. 5. 



FIG. 5a. 



In homogeneous light the central and lateral images 

 agree in size and form, but in white light the diffrac- 

 tion images are radially drawn out, with the outer 

 edges red and the inner blue (the reverse of the ordi- 

 nary spectrum), forming, in fact, regular spectra, the 

 distance separating each of which varies inversely as 

 the closeness of the lines, being, for instance, with the 

 same objective, twice as far apart when the lines are 

 twice as close. 



The influence of these diffraction spectra may be 

 demonstrated by some very striking experiments, which 

 show that they are not by any means accidental pheno- 

 mena, but are directly connected with tne image which 

 is seen by the eye. 



The first experiment shows that with, for instance, 

 tlie central beam, or any one of the spectral beams 



