exposure " - indeed, any time can be reckoned which will be required to expose a photographic plate, actually an ortho-chromatic - so that a correct printing negative will be finally obtained. If actually La Baume Pluvinel's theory and tables of resultant times of exposure have only an approximate precision - which is to be expected from the impossibility of accurate measurement of the different constants which arise - still it will be useful to the reader to be made acquainted with the results of this theory, even without following the long mathematical calculations of the author, and therefore we recapitulate the principal results and empirical data of La Baume Pluvinel.
However, before the general final formula for the time-exposure is given, the units of the factors appearing in it should be ex-plained, or their mathematical expressions be given.
An important factor is the amount -
This is briefly designated In which e represents the thick-
ness of the gelatino-bromide of silver film ; m, its absorptive coefficient; v, the absorptive coefficient of the film saturated with reduced silver; a1 and a2 the angle which the secondary axes of the different illuminated elements of the object form with the principal axis of the objective; A1 and A2, the corresponding actinic active quality of light (reckoned for units of time, surface, and distance). One can now actually, for any single case, approximately reckon the value ofwith the help of certain
assumptions, and on the basis of actinometrical and other measurements. Still, this would be impracticable for practical
work ; it is thus better to estimatein a purely experimental
way for special requirements, and the different cases which occur in practice, from this to reduce (see the table of the
value ofbelow). This also applies to the amount J, which re-presents the actinic intensity of the illuminating beam of light, and for which one finds a table below. For the desired time of ex-posure, /, results now, according to La Baume Pluvinel's theory -