B. T. LOWNE ON INTERFERENCE PHENOMENA. 369 



critical observer knows lines crossing each other diagonally on the 

 valve are false, not true images. They are, however, Professor 

 Abbe's " Delineation of structure." 



A diffraction image is not necessarily a false image, but it is 

 impossible to tell which of the numerous diffraction images is the 

 true one ; therefore the critical observer uses conditions which 

 destroy the diffraction image. 



That the wide illuminating peneil which covers the aperture of 

 the objective destroys the diffraction image is easily proved ; and 

 theoretically it should do so. The greater the amount of light — 

 that is, the larger the cone of direct rays, as distinguished from 

 those which are diffracted — the less the effect of the diffracted rays. 



If we remove the eye-piece from the microscope we observe, 

 under such illumination, that the dioptric or central pencil occupies 

 the whole back of the objective, and that no diffraction spectra can 

 be seen. There is no doubt they exist, but they are too feeble to 

 produce any visible image. This is the condition of illumination 

 insisted on by all who have worked at critical images of diatoms. 



Critical images such as Mr. Nelson and Mr. Smith will show you, 

 exhibit none of the phenomena which are always present with 

 diffraction images. 



The diffraction images alter with an altered focus ; the critical 

 image disappears ; it has one sharp focus and one only. The dark 

 or bright lines seen, which are really diffraction images, with a 

 Noberts' plate or lines on silvered glass, have no individuality; for 

 example, a diamond scratch on glass has splintered edges, it is an 

 irregular furrow. The diffraction images are sharp lines, quite 

 unlike the real lines existing on the glass. In the critical image 

 of the diatom on the other hand, each opening has an individuality 

 of its own. Spicules project from its margin, it is smaller or 

 larger than the average ; lines of fracture are distinct ; all appear- 

 ances well known and insisted upon by the critical worker. Lastly, 

 the diffraction image disappears with a full pencil of light with 

 the diffraction spectra, whilst the critical diatom image can only be 

 obtained by such illumination. Everyone now admits that the 

 illuminating pencil must be direct and not oblique, whilst, as Abbe 

 has distinctly shown, the oblique pencil gives the best results so 

 far as diffraction images are concerned. 



To test this question I broke up a globule of mercury, with an 

 elastic stick, upon a glass slide, and mounted the preparation in 



Journ. Q. M. C, Series II., No. 24. 28 



