630. Small particles of dust on the sensitive film of the unexposed plate will, if allowed to remain there, retard the action of the light when an exposure is made, so that when the plate is placed in the developer no action will take place in this particular spot. This leaves a transparent hole in the emulsion, which will be black in the print. These spots are termed pin-holes.

631. The advanced worker does not view a pin-hole or two with any great degree of consternation. He knows they are sure to occur at times, no matter how much care is taken to avoid them. One should, of course, take precautions, for prevention is far better than cure. The camera and all other apparatus, as well as the dark-room, should be in such a condition of cleanliness that there will be little or no cause for pin-holes.

632. Keeping Apparatus Clean

Keeping Apparatus Clean. The interior of the camera should be dusted out occasionally with a damp cloth, or one slightly moistened with glycerine. Care must be taken, of course, not to leave the instrument damp. The glycerine will, therefore, be a more satisfactory medium to employ than water. The glycerine will also pick up many particles which the dampened rag would leave untouched. Plate-holders, changing-boxes, or roll-holders should also be carefully dusted. When inserting the plate in the holder, and withdrawing or inserting the slide, there will be danger of dislodging particles of dust and causing them to adhere to the sensitive surface of the plate if the holder and slides were not thoroughly dusted.

633. The folding types of cameras have ventilation apertures at the corners of the ground-glass, so when focusing the air may be drawn in or squeezed out. The focusing-magazine type of cameras, however, are not so ventilated, and when focusing them the air finds its way through minute holes with considerable force. These small, but powerful, currents of air stir up whatever dust may be present, and the particles are bound to locate on the surface of the plate.

634. The careful dusting of all plates is to be commended, but unless this is done slowly you are apt to electrify the film and cause more dust to collect than you remove. The conscientious worker is always extremely particular regarding the dusting of a plate, but the practical worker may not even possess a dusting brush for this purpose, knowing full well that dusting a plate may easily put more dust on it than it removes. It is sometimes recommended that the dust be blown off of the plate, but this is not advisable, as particles of moisture are very likely to be blown from the lips on to the plate. The most effective method is to simply give the edge of the plate a sharp tap on the bench or shelf, holding the plate at one end and striking the wooden shelf with the plate, meeting the surface at right angles. Any particles of dust which could be removed by means of a brush will be dislodged by this knock.

635. The brush very easily electrifies the surface of the film and very often causes dust in the atmosphere, as well as any particles that may be on the brush itself, to adhere to its surface. After all, when the plate is taken from the box there is very little danger of there being any dust on it, for the plates have been handled in a perfectly clean room at the factory and packed in absolutely dust-proof boxes.