1 66 PHOTOMICROGRAPHY 



1. Non-colour Sensitive or Ordinary Plates. Sensitive 

 to violet and blue only. 



2. Orihochromatic Plates. More or less sensitive to 

 green and yellow as well, and made by incorporating 

 certain red dyes, such as erythrosin, in the emulsion used 

 for ordinary plates. 



3. Non-screen, Non-filter, Auto-screen, etc. Orthochro- 

 matic plates which have been further dyed in such a way 

 that their blue sensitiveness is reduced. They give the 

 same effect as an orthochromatic plate used with a pale 

 yellow screen to cut off some of the blue light. 



4. Panchromatic Plates. As nearly as possible sensi- 

 tive to the whole spectrum, and made by bathing the 

 finished non -colour sensitive plates in certain dyes, for 

 example, the isocyanines. 



In many cases the enhanced sensitiveness of the plate 

 to colours other than blue will suffice in itself to adjust 

 the contrasts in the negative. And generally, when there 

 is very little colour difference or contrast between parts 

 of the object or between the object and background, a 

 panchromatic plate is very much superior to any other 

 kind for recording these differences, and giving a negative 

 with detail and contrast. Even an orthochromatic plate 

 shows to great advantage over ordinary plates in these 

 and other cases, but to a less extent than a panchromatic. 



The four photographs on Plate 21 will serve to show 

 how very dependent the proper realisation of contrast 

 between differently coloured parts of an object is on the 

 character of the plate used. They were taken in the way 

 described on page 128, with a 1" anastigmat lens. 



There are, therefore, three factors to be considered 

 simultaneously : 



The constitution of the light emitted from the radiant. 



The transmission of the screen. 



The colour sensitiveness of the plate. 

 So that to eliminate the blue rays, for instance, in the 



