Tlic Helmholtz Theory of the Microscope. By J. W. Gordon. 403 



question really is, Will it not be more impaired by diffraction ? 

 What about diffraction that starts in the objective ? What about 

 diffraction that starts from the stage itself ? What about diffrac- 

 tion that starts from various points along the beam in its course 

 from the source of light to the observer's retina ? 



It will strike you probably that to a certain extent these ques- 

 tions answer one another and themselves. Take the last for 

 example and in a simplified form put it with reference to a con- 

 verging beam of light which focusses to a point as shown in 

 fig. 90. Observation shows that we obtain visibly the same dif- 



Fig. ito. 



fraction pattern whether we limit the beam by the large aperture 

 A x> or by the smaller aperture A 2 , or by the still lesser A 3 , provided 

 the one is the optical projection of the others along the course of the 

 beam. The distance from rj to rj r is the same however far away 

 the diaphragm may be, provided it always subtends the same angle 

 at the focal point rj. 



The geometry of this relation is somewhat intricate and involves 

 too much elaboration to be developed here. Helmholtz turns the 

 difficulty very neatly by tracing the course of a diffracted beam 

 through an optical instrument. The following diagram (fig. 91) 

 will illustrate his argument. 



Assume an object point at e radiating light through the aper- 

 ture A ... A, which light is focussed by the refracting system 

 B at 7], No assumption need be made about this refracting system 

 except that it is aplanatic and produces in the image plane i] . . . rj x 

 a correct image of the object in the object plane e . . . e^ Then, in 

 the aperture a diffracted beam will take its rise, which, being 

 refracted as an oblique pencil, will be brought to focus at, let us 



