SCEEENS PLATES EXPOSUKE 227 



indications of the relative value, and would depend on the 

 manner in which the illuminants are worked to ensure getting 

 the greatest efficiency. 



The speed of plate will, within wide limits, also influence 

 the time of exposure ; but here again it will be wise for one 

 make and variety oi plate to be adopted and adhered to. The 

 speed-value of the plate is usually indicated by the makers 

 on the box; in fact some makers, particularly Messrs. 

 Wratten & Wainwright, now determine the speed-value for 

 each batch of plates sent out, so that marked speeds may be 

 taken a* indicating very exactly what is the actual speed of the 

 plate. Formerly, exposure had to be effected with a considerable 

 degree of accuracy to ensure that a negative with proper 

 gradation should be obtained ; but latterly, plates have been 

 made which admit of considerable latitude in this respect. 

 In microscopic work, as in ordinary photography, it may be 

 regarded as a safe rule to expose for the deepest part of the 

 subject that is, expose for the shadows. But in developing, 

 regard must generally be had for the high lights ; extreme density 

 must not be obtained in the high lights and detail in those parts 

 must not be obliterated before that in the shadows is brought 

 out. Exposure should be full in any case ; it is better to 

 over-expose than to under-expose. An under-exposed plate is 

 useless, and may be regarded as such at once without wasting 

 any time in endeavouring to save it; but an over-exposed 

 plate, in view of the latitude that now exists, may be suitably 

 developed and result in a good negative. 



No indication can be given as to the increase of exposure 

 necessary when using colour-screens. For ordinary photo- 

 graphy the yellow screen which is supplied commercially 

 increases exposure by daylight from four to ten times, depend- 

 ing on its depth. Should such a screen be used, the same 

 factor will to some extent apply, but will vary according to the 

 illuminant. Where screens of varying colours and varying 

 absorbing power are used, the only practicable way is to 

 determine by trial exposures what the actual increase really is. 

 A record may then be made of what this amounts to, 

 and allowance made for such increase when exposing. With 

 some screens the increase is not very great. Take for 

 instance the Gifford F-line screen, which is visually not very 



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