The Optical Magic Lantern Journal (July 1894)

The Optical Magic Lantern Journal and Photographic Enlarger. 119 Dust, _AmonG the many questions that we have received ‘from correspondents, we have one which says, *« How is it that when one looks at the nozzle of a lantern which is lhghted, he sees the rays of light diverging towards the screen on which the picture is seen. Ialways understood ‘that light could only be seen when it fell on ‘something, but in the case spoken of it falls on nothing.”’ Thinking the subject a good one, we have decided to devote more space to it than would have been the case had we answered it in the Notes and Queries column. To the asker of the question, it may not be known that tbe air contains in suspense a great number of small ‘particles of dust, and it is the reflection of the light from these minute particles which renders ‘the rays visible. The more dust there happens ‘to be about, the better can the rays or rather the reflection of them be seen. Were it possible in a room to remove the ‘small particles of dust spoken of, no rays would be seen in thepo sition mentioned ; to demonstrate this in a room would be an impossibility, ‘but we will give particulars as to how this may ‘be shown on a small scale. If a box containing glass slides and ends be prepared, and a layer of some sticky substance be spread on the bottom of the box inside, and the whole thing set to one side for a time, the dust contained.in the air which is inside of the box, will settle and become attached to the ‘sticky substance at the bottom. The dust having ‘settled; and if the corners, &c., of the box be made absolutely air tight, it follows that we will have ‘a vessel containing air which is practically free from atoms of dust. If such a box as that spoken of be inserted in the rays of light from a lantern lens, and looked through, none of _ the rays will be visible, for the air reason that there | will be no atoms in suspense from which the |; light can be reflected. Various computations have been made as to the number of particles of dust contained in so ‘small a space as one cubic inch of air, and one authority gives the number in one cubic inch of air in an ordinary London street at 1,640,000 particles, and states as his opinion that no ‘place boasts of less than 9000 to the cubic inch. In the illustration given above A, represents ‘the lantern nozzle, B the glass box in which the rays are invisible, and C as seen outside. Hand Cameras for obtaining Slides for the Lantern. No. 32.—TuHe ‘‘ Hawk-eye.” THe Hawk-eye Camera is a_ well-known instrument in the United States of America, and is fast becoming popular here owing to the efforts of the European Blair Camera Company, of 9, Southampton Street, E.C. With this camera either glass plates, cut films, or film roll holder may be used according to the fittings supplied. A tank is provided in the rear of the camera of sufficient size to contain four double holders for glass plates: six for films or a roll holder capable of containing up to one hundred exposures. For focussing, a small doorin the rear may be removed and a celluloid focussing screen used ; and two finders are supplied for snap-shotsTo use the camera, the telescopic front is racked out until the index shows the exact focus on a scale provided for the purpose, the view being seen in miniature on one of the finders, there being two, one for horizontal and the other for upright views. The shutter is set by moving a small lever at top ioto a notch and is liberated for time exposures by a lever at side, whilst for instantaneous work a knob at the top of camera is employed. With the roll holder the spool has ‘ merely to be dropped into its place when it is