100 SUMMARY OF CURRENT RESEARCHES RELATING TO 



If the diameter of a particle be called e, then we may assume that for 

 photographic resolution it is necessary that the linear distance between 

 the centres of the lines be equal to 4e. E. C. C. Baly* states that e 

 may be taken as lying between 0*005 to 0*025 mm. ;l this statement 

 is not confirmed, however, by other workers. It is not difficult to make 

 slow plates in which the grain does not exceed a diameter of * 0005 mm. 

 According to Wadsworth, these plates should therefore resolve lines 

 which are not much more than T -^q mm - apart. As rough experi- 

 ments showed at once that the resolving power of such plates did not 

 exceed about £$ to 4V mm., the author undertook to thoroughly 

 investigate the subject. After numerous experiments, he concluded 

 that : 1. The resolution of a photographic plate is dependent on the 

 amount of irradiation displayed by that plate. 2. That irradiation is 

 not directly proportional to the size of grain, but is caused by two 

 different forms of scatters arising from (a) reflection and (b) diffraction. 

 3. That the resolving power is likely to be much smaller than that 

 indicated by the theory of Wadsworth. 



In order to experimentally determine the resolving power, a series of 

 black and white line gratings were constructed having alternate black 

 and clear lines of equal width, the width of the clear glass ranging from 

 0*88 to 0' 14 mm. Experiments showed that the limit of resolutions 

 possessed by dry plates chemically developed were : For an ordinary 

 fine grained plate, lines will be just resolved if they are separated by 

 • 018 mm. (For a coarser grain, as in all fast plates, about ' 030 mm. 

 is necessary.) For very fine-grained plates for violet light, 0*018 mm. 

 will be resolved ; with red light, * 008 mm. may be discerned. The 

 resolution on the surface of a fine-grained plate will obviously be much 

 greater than this, as is shown by the very high resolving power possessed 

 by the fine-grained "albumen" plates which are developed by the 

 deposition of silver from an acid silver solution. 



Specially prepared gelatin plates of extreme thinness were also pre- 

 pared, and were found to be more sensitive to red than to blue. The 

 separation with violet light was * 008 mm. ; while with red light lines 

 of * 004 mm. separation were resolved. 



[Photomicrographers will readily appreciate the superiority apparently 

 possessed by red light and medium grains over violet light and fine grain 

 plates. In resolving line tests the former go nearly twice as far as the 

 latter. — Ed.] 



Ultramicroscopic Cinematography of Living Microbes and of 

 Moving Particles.f — For carrying out the above purpose, J. Oomandon 

 used as light-source an arc lamp of 30 amperes with automatic regulator, 

 the luminous rays being condensed by a thin glass lens in such a way 

 that the image of the positive crater of the arc covered the diaphragm 

 of the Microscope condenser. The Microscope (a Zeiss) was provided 

 with Siedentopf's parabolic condenser giving lateral illumination and 

 thus forming the ultramicroscope. The cinematograph was Pathe's 

 apparatus modified for the purpose, and adapted to the Microscope by 

 the help of a bellows (soufflet). The movement of the film was 



* Spectroscopy, p. 339. 



t Comptes Rendus, cxlix. (1909) pp. 938- 41 (1 pi.). 



