216 PKACTICAL PHOTO-MICKOGKAPHY 



with a given objective is to multiply the N. A. by twice the number 

 of waves to the inch of the illuminant. It therefore follows 

 that something approximating to twice the resolution may be 

 obtained if light of short wave-length such as violet be used 

 instead of, say, the red at the opposite end of the spectrum. 



This method is practically only applicable when attempting 

 the resolution of difficult diatoms, or objects with similar 

 recurring structure. In practice, the increased resolution 

 rarely works out at such greater efficiency as theory 

 would indicate, and in general the use of a monochromatic 

 green, or blue-green, light will secure nearly all that is 

 required. 



The main object of colour-screens in photographic work is 

 to increase contrast. For example, if we have a faintly stained 

 methylene-blue preparation, and we attempt to photograph 

 it on an ordinary plate without a screen, although the visual 

 contrast is considerable, the resulting photograph, owing to the 

 actinic power of the blue light transmitted, will be disappointing. 

 To obtain a photograph of such a preparation therefore, recourse 

 would be had to the use of an orange-colour or deep-yellow 

 screen. This would effectually block out the greater portion 

 of the blue transmitted, and the image formed would therefore 

 be almost black in appearance, and in the resulting photograph 

 would give the maximum obtainable contrast. 



To obtain the maximum contrast effect with any particular 

 colour, the simplest way is to ascertain by means of a direct- 

 vision spectroscope what its absorption-spectrum actually is, 

 and to use a screen which as far as possible only transmits light 

 in the particular part that the object absorbs. The example 

 mentioned of a methylene-blue specimen "with which an orange 

 screen is used, fully carries out these conditions, except that 

 methylene-blue transmits a narrow band in the red ; but 

 unless a plate sensitive to the whole spectrum is being employed, 

 this is not a matter of moment. 



An example in the opposite direction might be the case of 

 a section or specimen stained with eosin, which would visually 

 be of a pale red or pinkish colour. If this preparation were 

 actually only transmitting red, then a blue screen would be of 

 use ; but on observation it will be found that it transmits a 

 great deal of blue as well, so that there are two colours 



