The Optical Magic Lantern Journal (August 1889)

22 The Optical Magio Lantern Journal and Photographic Enlargor. Discs and their Relation to the Lens. WHEN one is called upon to give a lantern entertainment in a hall or room, the following questions will be uppermost in the mind of the operator :—(1) What size of disc can be obtained with a lens of a certain focus ? (2) How far distant from the screen must the lantern be placed in order to get a disc of a certain size with a given lens of ascertained focus? Doubtless many more questions will arise, but these mentioned will be of the most importance. Itisa “rule of thumb” _ practice for an operator to wheel his apparatus up and ‘down aroom in order to find the desired position from ._ which to officiate, and the minds of any spectators will not be confirmed in the idea that the exhibitor thoroughly understands his business. How very much more simple and satisfactory is it to reason thus before starting for the place of entertainment. A screen of —ft. diameter is required, so if I bring a lens of — focus the lantern must be —ft. from the screen, the length of the room being of course taken into consideration in order to ascertain that it is possible to erect the lantern at the desired distance. Thishaving been ascertained beforehand, all that is required is to take an objective of the desired focus and measure off the necessary space between the screen and the place where the lantern should be set. Supposing we are called upon to operate the lantern in a hall 25ft. in length, we first ascertain the size of disc desired, which we will suppose to be roft. With an objective having a focus of 6in., how far from the screen must the lantern be placed in order to produce a loft. disc ? Here is the rule by which it can be ascertained. Let A=focus of objective. » B=diameter of slide. » C= ” » disc, » D=distance between the lantern and screen. Multiply the diameter of the circle required (C) by the focus of the lens (A) and divide by the diameter of the slide (B). CX A_p—10 x 6 B It is thus seen that in order to produce a Joft. disc with a Gin. objective, the lantern must be placed 2oft. 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 positions wnich we will suppose to be just 2oft. apart, and that the diameter of the disc must be roft. 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 =a focus of lens= =20 feet. 20 X : 20 3 —6in. focus. Again: We have a lens of 6in. focus, and intend that 20 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 by the focus of the lens used (A), which gives us D x Bic size of disc—2 = 3 =Ioft. diam. of disc. The following Ready Reference Table has been computed by the foregoing rule, and by a glance it will show the relations between lantern and disc with object-glasses of every focus from 4in. to 15in. READY REFERENCE TABLE. Distance ; FOCUS OF LENS. erat tin. din. Gin. Tin. Sin, Yin. 10in. = Llin. 12in. 13in. — 14din. 1din. antern and Screen. DIAMETER OF DISC. ft. in.| ft. in.) ft. in.] ft. in.) ft. in.j] ft. in. ft. in|] ft. in] ft. in| ft. in: ft. in.) ft. in, 10 feet. 7 6 6 0 5.0 43 389 3 4 3 0 2 9 2 6 24; 2 2 2 0 Ml» 8 3 6 7: 5 6 49 + 2 3 8 364 3 0 29 2 6 2 4 2 2 12 9 0 7 2; 6 0 5 2 + 6 40:3 7 3.3 3.0 2 9 2 7 2 5 | 13 ,, 9 9 7 10 6 6 6 7 411 +4 3 11 3.67 3 3 3 0 29 27 14, 10 6 8 5 7 0 6 0 5 3 + 8 4 2 3 10 3.7 3 3 3 0 29 1b: 4; ll 3 9 0 7 6 6 5 5 8 5 0 + 6 4 1 3.9 3 6 3 3 3 0 20 4, 15 0/12 0:10 0 8 7 7 6 6 8 6 0 5 6 5.0 4 7 £ 3 4.0 25 4, 18 9/15 0:12 6);10 9 9 + 8 4 7 6 6 10 6 3 5a 9 5 4 5 0 130 ,, |22 6/18 0:15 0/1210/11 3/10 0 9 0| s 2! 7 6] 611] 6 5] 6 O 35, 26 3; 21 0/17 6]15 0/13 1411 8 10 6 96; 8 9, 8 I 7 6 7 0 40, 30 0] 24 0,20 0] 17 2] 15 0/13 4 12 0]1010;10 0; 9 2 8S 6 8 0 45 wy, 33 9! 27 0; 22 6/19 3/1610!15 0 13 6]12 3/11 3:10 4 9 $ 9 0 50 .,, 1: 37 6/30 0| 25 0] 21 5}; 18 9 16 8 15 0/13 8] 12 6'11 6 109/10 0 Exampces.—An Sin, focus lens at a distance of 35ft. will give a disc of 13ft. lin. To produce a disc of 12ft. with a lens of 10in. focus, the lantern and screen must be separated by 40ft. To produce a disc of ldft. at a distance of 45ft. will require a lens of 9in. focus.