368 B. T. LOWNE ON INTERFERENCE PHENOMENA. 



but at higher and lower focal image planes false images are pro- 

 duced. As the black lines forming the image depend entirely on 

 the different paths of the two beams, they are independent of the 

 structure producing them, and give, as Professor Abbe justly 

 observes, no indication of real structure. 



It may not be quite clear how the dioptric and diffraction pencils 

 interfere. This depends on the fact that the diffraction pencil 

 consists at its origin in the object of exactly the same waves, and of 

 waves in exactly the same phase as the direct or dioptric pencil, from 

 which it is split off by diffraction. All the conditions of interfer- 

 ence are present. In all this I agree, I believe, entirely with Pro- 

 fessor Abbe, and if this included all the phenomena, Professor Abbe 

 and his supporters would be entirely right in saying that the search 

 after true images is futile whenever the objects are very small. 



Every practised microscopist knows the false diffraction images 

 well enough ; they present a peculiarity which at once differentiates 

 them from true images, such as can be called critical. They alter 

 in a continuous manner with every small alteration of the focus. 

 By altering in a continuous manner, I mean that dark circles, 

 rosettes, eye spots, etcetera, enlarge and contract ; points open out 

 into rosettes or rings, rings or rosettes close up into points. 



Anyone may convince himself of this if he places a diatom on 

 the stage, and illuminates it from the condenser with a very narrow 

 pencil of central light, even with a coarse object like Triceratium 

 favus. A narrow pencil, obtained by closing the iris diaphragm of 

 the condenser, gives the diffraction images with great perfection. 



What, however, does the critical observer do ? He opens his 

 diaphragm so as to give an illuminating pencil equal in angular 

 divergence to the aperture of his objective. Do this, and all the 

 diffraction images vanish. 



Professor Abbe says : — " In general such a pencil entirely 

 destroys the delineating power of the microscope for very minute 

 objects and high angles of aperture." 



In proof of this the experiment with Noberts' plate is given as 

 an example. If you open the condenser diaphragm the lines dis- 

 appear. This shows that there is no image of those lines, except a 

 diffraction image. Such fine scratchings on the glass are really 

 invisible, but they give diffraction lines which are visible. The 

 structure of Pleurosigma is visible under conditions which render 

 the diffraction lines produced by the ruled test-plate invisible ; 

 therefore they are not to be classed in the same category. Every 



