712 SUMMARY OF CURRENT RESEARCHES RELATING TO 



were found to be spherical in ordinary plates, while isochromatic plates 

 of several makes showed the peculiarity of having elongated or spicular 

 grains at the surface of the film. These in passing downwards through 

 the film gradually gave place to rounded particles, until close to the 

 supporting glass these latter were the only ones found. Intensification 

 increased the size of the particles, and these also varied with rapid and 

 slow development. With rapid development the silver particles most 

 nearly approached the size of the original particles of the silver salt from 

 which they were produced. Prolonged development favoured enlarge- 

 ment of the particles by reason of the formation of " group particles " 

 as well as by accretion. 



On Suiting Contrast Screens for the Photography of Bacteria.* 

 E. J. Spitta commences his article by reminding his readers, that while 

 the eye is sensible to differences of colour, the photographic plate can 

 only perceive contrast. If, therefore, the images of two selected coloured 

 objects of equal brightness are thrown upou a plate — provided it is 

 specially prepared to be equally sensitive to both colours — their effect 

 upon the emulsion is precisely the same, and their images appear similar 

 within certain limits. But if it is desired to increase the contrast 

 between two colours in photomicrography, one colour must be made less 

 bright than the other. The object of the author's paper is to discover 

 suitable screens for producing this effect. If the spectral colours of 

 red, orange, yellow, green, blue, and violet are thrown upon an ordinary 

 photographic plate (Plate XII.), it is at once seen that the emulsion is 

 not sensitive to the entire range of the spectrum, and that the different 

 colours which affect it do 'not do so equally with one another. This 

 selective capacity may be called the " eye " of the plate. It is known 

 that isochromatic or orthochromatic plates are those in which the sensi- 

 tiveness of an ordinary plate has been extended by staining the film 

 with some dye. Plate XII. gives a selection of " eyes " of several iso- 

 chromatic plates. The wave-lengths in /a/i are given the entire length 

 of the spectrum, whilst little linear demarcation-limits are also placed 

 (somewhat empirically chosen) where one colour may be said to merge 

 into its neighbour. Inasmuch as all plates have a cumulative power, 

 so with a long exposure (say 15 to 20 seconds) one part of the spectrum 

 seems as it were to catch up the other parts ; hence the final effect 

 appears very similar in many cases, so far as relates to density, although 

 differing in distribution along the spectrum, one part appearing to be 

 affected more than another. This apparently equal density, as a matter 

 of fact, is more apparent than real, for each emulsion, in reality, is 

 more sensitive to one or two particular wave-lengths of light than to 

 any others. This is the reason for supplying the extra column of ex- 

 ceedingly short exposures on the right hand of Plate XII., which shows 

 at a glance where the chemical action in each case seems to have com- 

 menced. It will be observed (Plate XII.) that : 



1. In the Edwards Iso-mediwn plate, action begins between wave- 

 lengths 525 and 570, that is in the yellow : whilst with fairly long 

 exposure its sensitiveness reaches to about GOT in the orange and to 

 nearly the extreme end of the violet. 



* Photography, xvii. (Juue 25, 1904) pp. 577-D (4 plates). 



