PRACTICAL PHOTOMICROGRAPHY, 95 



the course of development as a rule sufficient bromide is 

 evolved (beyond the soluble free bromide we put into the de- 

 veloper), to greatly retard if not to arrest the growth of detail 

 and density. Where the subject itself presents violent contrasts 

 it is well to expose to such an extent that the first dose of 

 developer shall reveal all detail without undue density in any 

 part. Where the high lights or actinic colors are, as above 

 advised, deliberately over-exposed in such subjects, the action 

 known to photographers as the "reversing action of light" 

 comes in, and these over-exposed high lights in place of being 

 densely black in the negatives, undergo the reversing action 

 and refuse to develop density. This, of course, suits us ad- 

 mirably in the cases under consideration of violent contrasts. 

 But if the subject be an ordinary one without violent contrasts, 

 or if we require as nearly as possible a black and white render- 

 ing of such objects as diatoms, then we find that when detail 

 is all " up " density is insufficient, and we reinforce our de- 

 veloper with alkali. In the case of the carbonates, time will 

 produce the desired effect, because though retarders are being 

 evolved there is no volatilization of the alkali ; but with am- 

 monia as the alkali, not only is fresh bromide being evolved 

 but the original alkali is evaporating, so we generally add 

 about 1^ minim of ammonia (15 minims of our 10 per cent.) 

 to each ounce of developer as soon as there appears to be a 

 halting in the acquisition of detail or density. 



To know when to stop development is a very serious matter. 

 Many so-called rules have been laid down, and many hints 

 given on the point, but we have never found any such rule or 

 hint to cover many cases. The image is not fully developed 

 as a rule until some part of it is visible from the back of the 

 plate, any further examination of the back may inform us of 

 the nature of the gelatine, of the nature of our subject, of the 

 degree to which the emulsion has been " cooked," but that is 

 all. If we have a standard non-actinic light, through the col- 

 ored medium guarding which we can see the flame through 

 the plate, we shall by experience come to judge very fairly 

 whether a plate of a batch we know is sufficiently developed. 

 The flame should be barely discernable as to shape through 



