The Moving Picture World (1907)

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THE MOVING PICTURE WORLD. 53 nost i. ms will arise, but these mentioned will be of the mportance. It is a "rule of thumb" practice for an jpera or to wheel his apparatus up and down a room in irder to find the desired position from which to officiate, ind tie minds of any spectators will not be confirmed in [he i'iea that the exhibitor thoroughly understands his iusin -SS. How very much more simple and satisfactory t is to reason thus before starting for the place of enter- ainment: A screen of — feet diameter is required, so jf I bring a lens of — focus the 'lantern must be — feet rom the screen, the length of the room being, of course, aken into consideration in order to ascertain that it is jossible to erect the lantern at the desired distance. This saving been ascertained beforehand, all that is required i to take an objective of the desired focus and measure iff the necessary space between the screen and the place there the lantern should be set. Supposing we are called upon to operate the lantern in hall twenty-five feet in length. We first ascertain the at of the disc desired, which we will suppose to be. ten eet With an objective having a focus of six inches, low far from the screen must the lantern be placed in rder to produce a ten-foot disc ? Here is a rule by which it can be ascertained. Let A = focus of objective. Let B = diameter of slide. . Let C = diameter of disc. Let D = distance between the lantern and screen. lultiply the diameter of the circle required (C) by the ocus of the lens (A) and divide by the diameter of the lide(B). 10X6 = D =..—■—— = 20 feet. It is thus seen that in order to produce a ten-foot disc with a six-inch objective, the lantern must be placed twenty feet from the screen. On the other hand, we may possess several lenses of different foci, and it is necessary that the screen and the lantern must occupy certain positons which we will sup- pose to be just twenty feet apart, and that the diameter of the disc must «be ten feet. How are we to ascertain whether we must use a lens of 4, 5, 6, 7, or other number of inches in focus ? Multiply the distance between the lantern and the screen (D) by the size of opening of slide (B) and divide by the size of disc (C). DXB 20X3 = A focus of lens = = 6 inches focus. C V jo Again: We have a lens of six-inch focus, and intend that twenty feet shall intervene between the lantern and the screen, and wish to know what size of disc can be produced- In order to calculate this it is necessary that we multiply the distance between the lantern and the screen (D) by the size of slide opening (B), and divide DXB by the lens used {A), which gives us = C size disc = 20X3 = 10 feet diameter of disc. CXA B ■ ■ ■ ■ v . ■ ■ 1 s :.- : : ■ . The Ready Reference Table following has been com- puted by the foregoing rule, and by a glance it will show the relations between lantern and disc with object glasses of every focus from four to fifteen inches. • : 4 in. Distance between' Lantern ■ r and Screen, ft in. 10 feet 76 11 " 5 in. 6 in. READY REFERENCE TABLE FOCUS OF LENS. 8 in. 9 in. I 7 in. 10 in. 11. in. 12 in. 13 in. 14 in. 15 in. DIAMETER OF DISC. 12 13 14 20 25 30 35 40 AS .50 8 9 9 10 II 18 22 26 30 33 3 o 9 6 3 o 9 6 3 o 9 37 6 ft. 6 6 7 7 8 9 12 15 18 21 24 *7 30 in. 6 7 2 IO s o o 6 b o 6 o o ft. in. SO 5 6 6 7 7 10 12 *5 17 20 22 25 6 a 6 o 6 o 6 o 6 o 6 o ft 4 4 S s 6 6 8 10 12 15 17 '9 21 in. 3 9 2 7 b 5 7 9 10 o 2 3 5. ft 3 4 4 4 5' 5 7 9 11 13 IS 16 18 in. 9 2 6 .11 3 8 6 4 3 j o 10 9 ft in. 3 4 ft in. .3 o 3 4 4 4 5 6 8 10 11 13 15 8 o 4 8 o 8 4 b 8 4 o 16 8 3 3. 3 4 4 6 7 9 10 12 13 15 4 7 II 2 6 o 6 o 6 o 6 o ft 2 3 3 3 3 4 5 6 8 9 10 12 13 in. 9 o 3 7 10 1 6 10 2 6 10 3 8 ft in. 2 6 2 3 3 3 3 5 6 7 8 10 II 12 9 o 3 7 9 o 3 6 9 o 3 6 ft in. 2 4 2 6 ft in. 2 2 ft in. 2 O 2 3 3 3 4 S 9 o 3 6 7 9 6 11 8 1 9 2 10 4 11 6 2 2 2 3 3 4 5 6 7 8 9 10 4 7 9 o 3 3 4 5 6 6 8 9 2 2 2 2 3 4 5 6 7 2 5 7 9 o o o o 5 8 o 9 o 10 o EXAMPLES.—An 8-inch focus lens at a distance of 35 feet will give a disc of 12 feet with a lens of 10-inch focus, the lantern and screen must be <iisc of 15 feet at a distance of 45 feet requires a lens of 9-inch focus. a disc of 13 feet 1 inch. To produce separated.by 40 feet To produce a '.. ■•'. • - .- - ■ ■. • -. • ■ .