B. T. LOWNE ON INTERFERENCE PHENOMENA. 371 



and whenever it has a lower refractive index than the medium in 

 which it is mounted, except when it acts as a concave lens ; it is 

 brighter than the field whenever it has a higher refractive index 

 than the medium in which it is mounted, except when it acts as a 

 concave lens, i.e., it gives a positive image. 



Diatoms have a lower refractive index than balsam, and seen by 

 transmitted light should give, in the majority of cases at least, a 

 negative image. Such a negative image is always complicated 

 with diffraction images, and is only seen with object glasses 

 having a low numerical aperture. The dioptric image is neces- 

 sarily feeble, as the diatom permits much light to pass through it, 

 and delineation is only possible by means of diffraction images. 



The case is, however, very different with high angles of aperture, 

 and especially with immersion lenses ; the diatom image is then 

 positive ; it is brighter than the field. How can this arise? The 

 diatom is self-luminous, i.e., in the same sense that a piece of white 

 paper is self-luminous. Every point of the diatom radiates light, 

 and every point is an independent source of light, that is, the light 

 radiates independently from every point, the vibrations proceeding 

 in every possible phase at every instant, such light producing no 

 visible interference phenomena. 



The cause of the positive image is that the diatom is illuminated 

 from above, not from below. It is illuminated by reflected light 

 from the upper surface of the front lens of the objective. 



It is well known that the pencil of light which falls upon a 

 plate of glass is partially reflected chiefly from the surface of 

 emergence. This surface of emergence of the front lens is a con- 

 cave mirror, which condenses the reflected pencil upon the object. 

 A very simple experiment will convince the most sceptical of the 

 great illuminating power of the back of the front lens of an objective. 

 Take a black-handled pocket-knife, the smaller the better, with a 

 bright stud upon it, hold it up between the eye and a gas-burner, 

 near the source of light ; the stud is invisible. Take an ordinary 

 pocket lens of an inch focal length or thereabout, and without 

 moving the knife focus it upon the stud ; it will be brilliantly 

 illuminated. 



Any convex lens will give a brilliant inverted image of a flame 

 upon a small screen placed between it and the source of light, by 

 reflection from its back surface. Moreover, if we look at the lens the 

 virtual erect image of the flame seen on its back surface is nearly 

 as bright as the source of light, although, of course, much smaller. 



