434 LIMITS OF OPTICAL CAPACITY OF THE MICROSCOPE. 



interference lines belonging to the diffraction image of another 

 slit which was cut with the lines at a very small acute angle, 

 sufficiently narrow to produce the interference lines at the point of 

 this angle. Eut these did not disappear when I threw an optical 

 image of the incident light on the plane of the double (parallel) 

 slit. In this experiment not the slightest suspicion could be 

 entertained that chromatic, or spherical aberration had dispersed 

 the rays over an interspace of 1 m.m. width. The only explana- 

 tion I can offer, is, that the light from the lens which 

 passed through the acute angle of the slit serving here as 

 object, suffers so strong a diffraction that it subsequently reaches 

 the two openings of the doubly-slit card with a corresponding 

 wave- phase and therefore sends interfering bundles through 

 both openings. In order to be able to see the interference lines, it 

 is necessary that their minima shall appear at a wider distance 

 from each other than the width of the lines of which they are 

 images, and when this condition is fulfilled theory does in fact 

 shew that the central clear portion of the diffraction figure of the 

 simple slit forms a line of light which is broader than the distance 

 between the two slits of the doubly-slit card. 



Similar relations take place (although more difficult to subject to 

 calculation,) when the fine edge of a dark screen is used as the 

 object. It is known that from such an edge, bundles of interrupted 

 rays (in linear formation) likewise bend themselves into the dark 

 field, which have corresponding phases of movement, and so when 

 bent by a second screen can exhibit regular interference. That 

 the resultant effect cannot become nil, appears clearly from the 

 fact that the effect of a bright line may be represented as the 

 product of the action of two endless half-planes bounded by straight 

 lines the edges of which half-planes slightly overlap each other, 

 minus the action of an equally bright whole plane. As the latter 

 causes no interference phenomena, the bright line of itself could 

 not cause interference in any part of the field, unless each of the 

 half-planes also produced such interference. It folio w^s therefore 

 that the light bent away from a straight edge must also spread 



