Focussing presents some little difficulty. It is useless to attempt to focus the sitt,ers themselves. The best plan is to place a lighted candle on the seats which they are to occupy, and sharply focus that. The candle being removed, the sitters may then be carefully posed, when, if the lens is a reasonably good one, it will be found that they will be quite sharp in the resultant negative. The lamps should not be placed too low down ; C and D should be about five feet from the ground, E being about three feet. The combustion of the magnesium results in the production of a dense cloud of white smoke, oxide of magnesium ; this subsequently settles in the form of a very fine powder, but will not cause tho least damage to furniture. It is advisable not to make e second exposure until the fumes from the first flash have entirely disappeared. Many people disregard this caution, and then wonder why they get fogged, flat negatives.
Flash-light photographs may be produced without the aid of a flash-lamp by the following method :—A tuft of gun cotton, weighing about 6 gr., is loosely spread out on a piece of sheet tin, or on the back of an ordinary tea-tray, and its surface sprinkled over with powdered magnesium, from 15 to 20 gr. should be sufficient. This may be placed on the top of a pair of steps, and ignited by means of a long wax taper. This methcd is almost as effective as the lamp, the chief drawback being the great amount of smoke that is produced, which, although perfectly harmless, causes a very unpleasant effect when inhaled. Gun-cotton being a very explosive substance when compressed, should not be tightly packed in the tin or vessel containing it, and should only be stored in small quantities.
A third method of utilising the magnesium light for photographic purposes is by the use of what are known as flash powders. All mixtures of this class are of a highly explosive and dangerous nature, both in their use and in their preparation, and I strongly advise the reader not to attempt to either make or use them. I shall therefore not give any formula for making them in " Elementary Photography." Flash-light photography affords a means of securing photographs of dark interiors which it would be difficult, if not impossible, to secure by other means. As an instance of what may be done in this direction, I may refer the reader to the illustration, and which represents the interior of a wine vault in the City to which the light of day has never penetrated. The exposure was effected by burning a large quantity of magnesium and guncotton in the manner already described, assisted by an auxiliary flash-lamp, two assistants standing in recesses in the wall, also burning long strands of magnesium ribbon. Two negatives were taken, both of which proved fairly successful.
I have said nothing about development, for it usually presents no special difficulty, the plates, as a rule, being fully exposed and coming up without forcing, The only practical difficulty in taking flash-light portraits is in avoid ing flatness on the one hand and hardness on the other. This being overcome the rest is plain sailing.