DURATION OF EXPOSURE.
A print from an average negative, upon Elliott's Platino - Matt bromide paper, which is a very suitable one for a. beginner, should have an exposure of about. six seconds ; using a No. 5 Bray's gas burner.
remove card, and give three seconds again. Then the first part will have had 6 + 3 + 3 or twelve seconds, the second 3 + 3 or six, and the last three seconds. It frequently happens that the frame will have to be covered with tissue paper to diffuse the light. This must. always be done when there is to be dodging or vignetting. It has been found that ordinary tissue paper only allows one-third the amount of light to pass therefrom. If the exposure for a plain negative is known, the exposure for a vignette for same will, on account of its being covered with tissue paper, therefore be three times as much.
This is merely intended as a rough guide ; the exact exposure being only found by experiment. The best plan is
to make an exposure in three sections, behind the negative, giving each one twice the exposure of the preceding one ; or assuming first an exposure as correct, and giving that to the centre portion, with half that and double that to the outside parts. This may be effected without any special frame, by just covering up parts of the negative with a card, and giving the assumed exposure to the first third, half this to the second third, and the same to the last third. For example, suppose a negative is to be tested ; fill in paper and cover up two-thirds, as in Fig. 328. Now assume any exposure to be correct, say six seconds. Give this exposure, then turn off the light or shut off with shutter, and move the card, as shown in Fig. 329, giving half the assumed exposure, or three seconds ; then shut off light once more,