This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Pure Magnesium Powder. Pure magnesium powder ignited with a special lamp is generally employed for illuminating interior photographs where figures are not included. The number of grains of powder will depend entirely upon the size of the room, color of the walls, as well as color of the furniture, etc. As the magnesium machines usually hold a minimum of one ounce, the chamber may be filled and any portion of this amount required may be used. If the tube for blowing the powder into the flame is used, even illumination will be effected by blowing a few light puffs from different points.
535. Interiors of halls, churches, stores, and other public places, are made on exactly the same principle as the photographing of interiors of residences, the only difference being in the required amount of powder.
536. Where figures are included within the field of view, it will be necessary to use a flashlight compound instead of the magnesium ribbon or powder, as movement would result if either of the latter were employed owing to the prolonged duration of the flash. There are occasions where the magnesium powder may be employed on interiors where figures are included in the view, but in such cases the subjects must be informed that the exposure is not instantaneous and, therefore, a position should be given them which they can hold for a few seconds without movement. The magnesium machine should be held in the hand and by moving about from a position on the right to one on the left while lightly blowing puffs of magnesium into the flame, a very even illumination can be given. Never pass in front of the camera while the flame is burning.
Flash-Machines. Usually, where many figures are introduced, the regular flash-powder and a flash-machine will be found the most satisfactory, as it works instantaneously and there is no movement of any subject during the exposure.
538. When using the flash-machine on interiors, remember, that the larger the interior the higher you should place the lamp and, of course, the more powder will need to be used.
539. As each room, hall, store, church and other interior requires special handling and lighting, it is not possible to give detailed instruction regarding each case, but the general information contained in this chapter should be adequate to enable you to proceed in securing the very best of results. It is not advisable, of course, to attempt anything on such a large scale as this until you have used flashlight material in a small way, such as photographing the average room, a figure or even still-life subject. It requires the fundamental training which you will secure with these simpler subjects to acquaint you with the peculiarities of this branch of photography. For smokeless flashlight work on a large scale, see Vol. IX. .
540. After having procured the necessary material for making of flashlights, it is advisable to first experiment in photographing the interior of a room or some inanimate object - the latter is preferable. Light this object in exactly the same manner as you would a person. Carefully load the flashlight apparatus which you employ, and proceed to make the exposure. A proof print should be made from the final negatives, with a memorandum of conditions noted on the back, and these proofs then filed in the proof file.
When, to your satisfaction, you can make a good negative of the still-life subject, proceed to make a Plain Portrait Lighting of a person, carefully following the instruction given for lighting, etc. Go about the work carefully and you will be successful. After your first experiments you will have less fear of accident and will be able to produce some very pleasing results. Proof prints should be made of all your first experiments with complete data noted on the back of each and all proofs should be filed in your proof file for future reference.