W. M. GllAaAM — THE tHOtOGRAl'IlY OV blPl'EKA. 159 



For a three-fold magnification of the image therefore an exposure of 4 times 

 that foimd by the exposure meter for the unmagnlfied image will be re([uired. 

 Supposing then the exposure found by the meter were 2 seconds, the exposure 

 for a three-fold magnification would re(piire to be 8 seconds. 



But besides the increased exposure needed for the magnification, the use of a 

 yellow filter necessitates a still longer exposure. This factor varies with the 

 nature of the filter and of the plate. It will be found printed upon a green label 

 upon each box of panchromatic plates. For a K III filter it is about 5 times, 

 but it varies slightly with diflf'erent batches of plates, as will be observed upon 

 the labels. 



To find then the additional length of exposure necessitated by the use of the 

 K III yellow filter, the exposure of 8 seconds as found above must be midtiplied 

 by the filter factor, which for a K III is about 5 times. Therefore 8 x 5 = 40 

 seconds, and this will be the exjjosure required in the above example. 



All the necessary factors are included in the following formula, by which the 



exposure for any magnification under any conditions of light, aperture, &c., and 



with any filter can be calculated : — 



(the magnification +1)^ . n , .\ ci. r , 



^ ^ — ' ' X exiiosure meter factor x the filter factor. 



4 



Development. 



The use of a safe-light is without any advantage and unnecessary, as the plates 

 can be best developed in complete darkness with the aid of a stop-clock that 

 strikes the minutes. For development, the directions of the makers as regards 

 solutions, temperature and duration should be followed. The density of the 

 negative is a matter of some importance, as a denser negative is re(|uired to 

 give a good print upon P.O. P. than is required to give a good print upon gas- 

 light paper. 



It is well therefore before beginning development to decide upon the type of 

 negative reipired and set the clock accordingly. The time required for the 

 production of 3 types of negative is given upon the card enclosed in each box of 

 plates. The hyjDO dish should be placed in a box with a light-tight cover, so 

 that as soon as the plate has been developed and placed in the hypo bath it may 

 be covered and the light turned up. Otherwise the operator must sit in the dark 

 until the plate is completely fixed. 



Retouching. 



In the case of a panchromatic plate used with a suitable fitter, retouching is 

 not very often necessary. But in prints intended to be used for the production 

 of half-tone blocks the details should be as sharp and as distinct as possible, as 

 there is always a degradation of the definition in such reproduction. 



In some cases then the retouching of the negative, if done judiciously by an 

 entomologist, will so increase the distinctness of the leg and body markings of 

 the insect, as to add materially to the accuracy of the half-tone reproduction. 

 If injudiciously done the remedy is worse than the disease. In every case 

 extraneous spots should be removed with the aid of a little retouching medium 

 and a 4 H or 5 H lead pencil. 



