144 PHOTO-MICEOGEAPHY 



the violet ; intermediate lengths being productive of sensations we call yellow, yellow- 

 green, green, greenish-blue, blue and blue-violet. 



2. The ordinary photograph is taken by the violet ray, because the usual emulsion 

 is most sensitive to that particular wave length. Plates, however, can be stained to 

 be extra sensitive in any one or more colours, but even then with this extra sensitive- 

 ness, the action of the violet is always the strongest, -i.e., more precipitation of silver — 

 time for time — is produced with violet light than any other colour. 



3. Ordinary light, whether daylight, electric or lime light, consists of a blending 

 in certain proportions of rays of all wave lengths. 



4. An ideally perfect monochromatic screen is one that intercepts all other wave 

 lengths save the one it passes. A red screen, therefore, cuts off (probably converts into 

 heat) all other rays, save the red ones ; blue cuts off all but the blue, and so on. It is 

 difficult to obtain a really monochromatic screen, and impossible to obtain a glass ideally 

 perfect, save perhaps in the case of red glass. It must also be noted that perfection of 

 monochromatism also depends on the strength of the illuminant, that is to say, a screen 

 may be almost perfect with a weak light, but far from perfect with a strong one. 



5. The effect of placing two different monochromatic screens, each ideally perfect 

 and of the same intensity, over one another would be to cause blackness, such black- 

 ness being the more perfect the more intense the screens or the feebler the white 

 light — screens of differing intensity vary in their effect according to such variation, 

 by which is meant an intense red and a feeble green will allow a residuum of red — 

 a strong green and a thin red, a residuum of green, and so on. 



As ideally perfect monochromatic screens are obtainable with such difficulty, it 

 may be well to point out that red filters often pass a little yellow ; blue ones often 

 a little red ; green ones occasionally a little blue, and often red ; whilst yellow ones 

 are rarely pure at all, permitting red and green rays to pass to some considerable 

 amount. If, now, a blue passing red be placed over a good green, but one passing 

 a modicum of red, and pi ovided the light be intense enough, a dark, deep red residuum 

 is noticed instead of blackness. This is obviously caused by the green and violet 

 causing darkness, but not sufficient to annihilate the reds from each glass. 



6. In practice, owing, of course, to impurities, it is found that some colours are more 

 antithetical than others. Eed and green, for example, usually produce greater blackness 

 than red and orange, a fact which we shall presently explain is made use of at times. 



Now with these six precepts before us we hope to show how th^ photographer can 

 increase the contrast in his negative, and so, of course, in the print which is taken from it. 

 Let it be presumed he has taken an ordinary photograph of a red baciUus on a 



