106 PHOTO-MICROGRAPHY 



(A.) To increase the contrast between different coloured objects in a specimen, 

 partial monochromatic Ught is a great power in the hands of the photo-micrographer. 

 This is especially felt when deahng with bacteria, which are usually stained one colour 

 whilst other objects around, or the ground-work of the slide may be stained, another. 

 For example, the bacillus of consumption stains red, and the remainder of the sputum 

 in which it is found usually appears blue. If, now, a photograph be obtained in the 

 ordinary way, a poor, flat picture results, in which the bacillus, the central object of 

 attention, is only faintly seen. Visually, the slide may present plenty of contrast, 

 photographically, it does not. The photographer, in this case, would use a coloured 

 glass, either orange or green, the depth of the colour depending upon the require- 

 ments of the specimen. The effect of so doing is this : The orange or green glass 

 cuts off the red rays and makes them jet black, and as black does not affect the 

 plate, so the silver is not precipitated, and after development clear glass is left ni 

 the place of any deposit. This clear glass in the negative causes the print to become 

 black which increases the contrast as required. It is necessary, as the illumination 

 is green, that an isochromatic plate be used, and it is needless to state that the exposure 

 has to be increased — say about three times that of white light — because of the 

 illumination being by one colour only instead of by ordinary white light. This matter 

 is dealt with it in its entirety when speaking of the photography of bacteria later on. 



(B.) To reduce contrast, partial monochromatic light may sometimes be employed. 

 Suppose, for instance, a specimen where red blood-vessels are prominent, and one that 

 contains details in the background desirable to be photographed. If an exposure be 

 given for them, the rest of the specimen, which it may also be desirable to show, will 

 be too faint. Yet, on the other hand, if a sufficient exposure be given for the latter, 

 the details on the blood-vessels will be choked up by over-exposure and lost. If now, 

 a red glass be used, provided a red plate be employed, and sufficient exposure added, 

 the extra exposure will increase the detail where it is wanted, and yet not so much 

 affect the portions where it is not required. A flat negative is now obtained, but one 

 full of detail and having no severe contrast. See also p. 1 1 3- 



(ii) Complete monochromatic light can only be obtained by the use of 

 prisms as explained Chapter YI. Section I. ; but objections are shown in the same 

 place to this method. Troughs of coloured fluid may be substituted between the 

 condenser and the illuminant, and these are effectual although still not quite so pure 

 as when light of one colour is prismatically obtained. But it is to the use that we 

 refer in this place. Space will not allow to explain ivhy it is that the shorter the 

 wave length — that is, the nearer the approach of colour to the violet end of the 



