46 



PROGRESS IN MICROSCOPY 



direction of 6 are concentrated in the focal plane of the objective O^ 

 because they are all parallel. Were 6 changed to —6, i.e. were the 

 phenomena considered to be symmetrical in relation to a perpendicular 

 grating, nothing would be altered. The maximum of light would still 

 obtain at S'^ and be symmetrical in relation to the axis. More par- 

 ticularly, were O^H = 0, viz. the light diffracted in the direction of 

 the incident light (0=0) investigated, the result would be the same. 

 All the sht-originated vibrations in the direction of the axis (0=0) 

 are in phase and maximum light is produced at 5*,', principal focal 

 point of the objective O^. The slit-originated vibrations are no longer 

 in phase when there are slight divergences in the directions relevant 

 to OiH = 0, A, 2A. Some are leading, some lagging and, in the ag- 

 gregate, they virtually cancel themselves out. There is virtually no light 

 as soon as the directions, requiring a path difference amounting to 

 a whole wave-length multiple, are diverged from. Lastly, when ob- 

 serving the focal plane of the objective O^ (Fig. 1.55) the maximum 

 of hght is at S^ on the axis and, symmetrically, on either side of 5,',, 

 a series of maximums S[, 5*0, etc. These maximums are images of the 

 source. The image S^ is the direct image of the source and unages 

 5*1', 5*2, etc. the diifracted images, called spectra. 



In the computing procedure originated by Abbe, forniation of the 

 imaged grating in the plane // is investigated in two stages: first. 



Y "^'^i^vl/o, 



C " ^- 



Fig. 1.58. All the spectra .S'l,^',,"- ^fe not (raiismitted hy the objective O^. 



formation of the images diffracted in the focal plane of the objective O, , 

 and next, the change-over from the focal plane to the plane P' where 

 the ultimate image is located. The light diffracted by the grating which 

 penetrates in the objective O^ is distributed in the images .S,',, S[, So, etc. 

 These images act as in-phase radiants. When the slits of the grating 

 are very close, spectra ^j, ^o, etc., are far removed from 5"^. To 

 collect the light diffracted in the direction of 6 the objective N. A. must 

 be adequate. If such is not the case, it may occur that all the spectra 

 *S(, ^2, etc., are stopped short by the objective O, (Fig. 1.58). Only 



