W. M. GRAHAM—THE PHOTOGRAPHY OF DIPTERA. 159 
For a three-fold magnification of the image therefore an exposure of 4 times 
that found by the exposure meter for the wnmagnified image will be required. 
Supposing then the exposure found by the meter were 2 seconds, the exposure 
for a three-fold magnification would require 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 different 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 IIT yellow filter, the exposure of 8 seconds as found above must be multiplied 
by the filter factor, which for a K III is about 5 times. Therefore 8 x 5 = 40 
seconds, and this will be the exposure 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 :— 
ie Be SEE x exposure meter factor x the filter factor. 
DEVELOPMENT. 
The use of a safe-light is without any advantage and unnecessary, as the plates 
‘an 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 required 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 required 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 hypo 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. 
