With regard to the size of the disc produced by the lantern, the following remarks and table from the Optical Magic Lantern Journal will be found of very great use: 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: i. 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. It is a "rule of thumb " practice for an operator to wheel his apparatus up and down a room 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 — feet diameter is required, so if I bring a lens of — focus the lantern must be — feet 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. This having been ascer tained 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 twenty-five feet in length, we first ascertain the size of disc desired, which we will suppose to be io feet. With an objective having a focus of six inches, how far from the screen must the lantern be placed in order to pro duce a ten-foot disc ? Here is the rule by which it can be ascertained: Let A=focus of objective.
" B=diameter of slide.
C= •4disc.
" 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).
C X A io X 6 =D=—=20 feet. B 3 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 positions which we will suppose 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 four, five, six, seven, 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).