SUMMARY OF CTJRREHT RESEAECHES RELAT3HG TO 



Dr. J. Pelletan refers* to the price of the objective as being 

 10,000 francs, or 400/., but we have no verification of this statement. 



Apocliromatic Objective stolen. [" From the K. meclianisch-technisclxen Versucha- 

 Austalt iu Berlin- Charlottenburg has been lately stolen an ai oehromatic objec- 

 tive of Call Zeiss of Jena, homogeneous immersion, numerical aperture 1"30, 

 focal length 2 mm. Besides the name of the firm and tlie usual data, the objec- 

 tive has the mnker's number 555 engraved in small figures. It is requested that 

 the objective may be retained should it be offered for sale."] 



Central- Ztg. f. Optik u. Mechanik, X. (1889) p. 143. 



Hetjrck, H. van — La nouvelle comhinaison optique de Zeiss et les paries de 

 I'AmpMpleura. (The new optical combination of Zeiss, and the beads of 

 AmpMpleura.') Bull. Soc. Belg. Micr., XV. (1889) pp. 69-71, 



C5> Microscopical Optics and ManipxilatioH.. 



Diffraction Theory.— Prof. B. T, Lowne and Mr. E. M. Nelson 

 have been in controversy on this subject, the former attempting to ex- 

 plain the phenomena of microscopic vision on a dioptric basis, while 

 the latter supports Prof. Abbe's views. 



Prof. Lowne explains as follows | the advantages arising from the 

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

 respectively. 



" 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, 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 necessarily feeble, as the diatom 

 j)ermits 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 as a piece of white paper is self-luminous. 

 Every point of the diatom radiates light, and every point is an inde- 

 pendent 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 concave mirror, which con- 

 denses the reflected pencil upon the object. A very simple experiment 



* Journ. de JNIicrogr., xiii. (1889) pp. 481-2. 



t Journ. Quek., Micr. Club, iii. (1889) pp. 360-72 (4 figs.). 



