370 B. T. LOWNE ON INTERFERENCE PHENOMENA. 



balsam. The slide contains thousands of globules of every degree 

 of fineness, many not more than half a micro in diameter, -guwo"". 

 Yet when illuminated with a wide pencil, covering the aperture of 

 the lens, all diffraction rings disappear with an oil-immersion lens 

 at the correct focus, and each globule, however small, is visible as 

 a small dark disc. It is easy, of course, to see the globules, like 

 cells with round nuclei, especially with objectives of small angle, 

 to get two rings round such a globule, or to make them appear 

 like transparent objects by altering the focus. Yet any critical 

 worker knows at once he has diffraction images to deal with, and 

 he alters his focus and illumination until he gets the definite un- 

 alterable image. 



With a glass having a small aperture it is impossible to get rid 

 of the diffraction images in all cases, but when the aperture is large 

 I believe this can always be done. 



As in most cases of controversy amongst scientific men, with 

 which I am acquainted, the disputants are talking about different 

 things. What Professor Abbe regards as " delineating power " 

 and " detail " is detail which no critical observer of diatoms would 

 recognize as delineation. Professor Abbe, working under different 

 conditions, regards a diffraction image as detail. No one supposes 

 that the lines of Noberts' plate are themselves visible. 



That diffraction may and does show us objects that we could not 

 possibly see without it is undoubted, and such objects must be 

 interpreted as best we may, but that the diatom structure is one of 

 these cases is certainly not true, and when you have a critical 

 image such as Mr. Smith or Mr. Nelson can show you, you will 

 find no diffraction spectra are visible, as the whole aperture is 

 flooded with light. 



The questions naturally arise, if this be true what have we in 

 the place of a diffraction image ? and what advantages arise from 

 the use of lenses with a large numerical aperture and of immersion 

 lenses respectively ? 



I shall now endeavour to answer these questions. 



IV. Positive and Negative Images. 



The images seen with the microscope are either brighter or 

 darker than the illuminated field. An opaque object appears 

 black ; when illuminated from below it gives a negative image. 

 A transparent object seen by transmitted light is less bright than 

 the field, i.e., gives a negative image, whenever it absorbs much light, 



