ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 807 



will convince tlie most scej^tical of the great illuminating power of the 

 back of the front lens of an objective. Take a black-haudled 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. 



With objectives of large numerical aperture, the working distance is 

 short, and with a large pencil much light is reflected upon the object. 

 With immersion lenses the reflection from the cover-glass and the front 

 of the objective is practically done away with, so that all the light 

 reflected from the upper face of the front lens falls uj)on the object. 



Five per cent, of the light which falls normally on the back surface 

 of a glass lens is reflected, whilst the quantity which is reflected by 

 oblique incidence rapidly increases ; much light is totally reflected, the 

 whole converges after reflection once, twice, or thrice towards the object, 

 and it must be remembered that only the centre of the pencil falling 

 upon the back surface of the front lens is transmitted to the eye, whilst the 

 whole pencil is concerned in the illumination of the object from above. 



I believe that this is the great advantage derived from high angles 

 of aperture, and more especially from immersion objectives. The 

 elimination of the false diffraction images resulting from the large 

 illuminating pencil, and the reflection of .light from the object, appear 

 to me to be the causes of the great increase of definition attained by their 

 use. The view propounded by Professor Abbe that they collect out- 

 lying difiraction pencils, appears to me quite inadequate to explain the 

 increase of definition." 



Mr. E. M. Nelson refutes Prof. Lowne's suggestion by the following 

 considerations : * — " One great objection to the dioptric theory is, that it 

 is unsupported by experiment. The single experiment put forward may 

 be said to touch the subject only in an indirect manner. I allude to 

 the reflex from the objective front, to which I shall refer later. 



(1) The point with regard to the images of the condenser diaphragm 

 at the back of the objective has nothing to do with the question. 



Let us take a simple case — viz. an oil 1/8 of large angle focused 

 on a'P. angulatum, illuminated by edge of flame, centered and focused by 

 stopped-down condenser on object in usual manner. Now, if we examine 

 the back of the objective we shall see the usual picture of the dioptric 

 beam and the six spectra round it. The size of the dioptric beam — i.e. 

 the disc of light at the back-lens of the objective — will depend on the 

 size of diapliragm and angle of condenser. The size of the spectra will 

 equal the size of the dioptric beam. If the object be now taken away, 

 we shall lose the spectra, but not the dioptric beam. Now, no one 

 imagines for a moment that this image of the diaphragm is projected to 

 a focus at the objective conjugate ; what is projected there is an erect 

 image of the edge of the flame. If the object be replaced, there will be 



* Eiig. Mech., xlix. (1889) pp. 425-6 (-1 figs.). 



