HIGH-POWEE, OR CRITICAL PHOTO -MICROGRAPHY 145 



white ground, with blue-stained nuclei of cells interspersed about. He finds a flat 

 result, a negative that produces a wretchedly poor print lacking all contrast. Now 

 let him place a fairly strong green screen over the illuminant and use a green-stained 

 plate to shorten exposure. The resulting negative will show clear patches of glass 

 corresponding to the red bacilli, a fairly clear deposit for the blue nuclei and an 

 intensely black one for the background. If the bacilli are too clear, so much so that 

 any little alteration of structure, such as segregation, seems lost, then let him use a 

 fainter green or employ a brighter light, and the result may be good. If still not 

 satisfactory let him try an orange screen, for as that colour is not so antithetical as 

 green is to red, it will allow a little more light to pass. 



The resulting print will give, of course, a black bacillus, a well-defined appearance 

 of the sca.ttered nuclei, while both bodies are seen lying on a clear white background. 

 The whole picture is full of contrast and pluck. 



As a matter of fact pure monochromatic screens for photo-micrographing bacteria 

 are not required, good pot glasses are amply sufiicient, for be it understood it is only 

 the increase of contrast that is required. 



To be practical, then, in photographing bacteria, glasses of all colours must be 

 procured, and also it is convenient to get several shades and several densities. For 

 what may give rise to sufficient contrast to the eye with the ordinary lamp, may not be 

 sufficient ; or, perhaps, on the other hand, which is much more frequent, may be too 

 great when the photograph is being taken. It would have been thought the intense 

 limelight should always demand a still greater thickness of contrasting colour, but it 

 is usually just the reverse, for the light is reduced considerably, as seen on the ground- 

 glass screen, with that visually viewed at the eye end. The very fact that a virtual 

 image is seen by the eye and an actual image viewed on the plate, entails a loss of 

 initial light, as the rays have to again cross. This crossing is obvious, as the image 

 is upside down to the eye, but the right way up on the plate in the camera. 



Screens may be used to increase definition (page 107), when taking photographs 

 of fine markings in diatoms, and the best method is to make them by mixing aniline 

 or other colours with a specially made collodion, formed by dissolving pure celloidine 

 in equal parts of ether and alcohol. Float the collodion over very thin glass plates — 

 cover-glass type of glass preferable, especially flat ones— allow to dry, and cover with 

 another protective glass, as when covering ordinary lantern slides. Malachite green, 

 methyl green, blue and violet aniline, fuchsine, naphthol yellow, chrysoidine, all of 

 differing quantities and densities, may be made ; they may be of service.* 



* "WJiilst these pages are passing through the press the following plan for making good screens has been suggested by 



