The condenser is the lens which collects the light and passes it through the slide to the objective. Condensers are of various forms and are mostly supplied of 4 inches or 4 1/2 inches diameter. For lanterns with a low-power illuminant the 4-inch condenser is best, for the light is concentrated on a smaller area, and consequently gives a more brilliant picture on the screen. Generally speaking, a 4-inch condenser is ample for all English slides, but 4 1/2-inch is necessary for American and French slides. In the best lanterns with limelight jets or arc lamps it is usual to use a condenser of the " Her-schel " form. This consists of a double convex lens and a meniscus lens mounted in a brass cell, the meniscus lens being turned to the light source. The convex lens allows air to get between it and the lantern slide and minimises the annoyance of moisture condensing on the slide during its exhibition. The focus of the lens should be adapted to the focus of the objective that is being used, but, generally speaking, one condenser is sufficient for lenses from 4 to 12 inches equivalent foci. When longer foci are required a longer focus meniscus lens must be fitted to the condenser, or otherwise the light would have to be brought so close to its surface that, even if the lantern construction rendered it possible, a broken condenser would probably be the result.
The lantern lens or objective is usually of the " Petzval " form, such as the old-fashioned portrait lenses, and it is admirable for the purpose. The larger the diameter of the lens the better so far as the brilliancy of the picture on the screen is concerned. When purchasing a lantern it is well to be prepared for eventualities and to get an instrument which can be added to from time to time. Focussing rackwork barrels for the front of the lantern are recommended, into which lenses of any foci can be slipped as desired. Really first-class lantern objectives of this class are not expensive, as will be seen from the following list : Brass Rackwork Focussing Tube to take lenses of various foci and 2 3/8 in. diameter back lens, complete with flasher and slot for tinting colours ........£1 o o
Lenses to fit into such tubes of any of the following foci,
7 in., 8 1/2 in., 12 1/2 in., 14 in., 16 in. and 18 in. each o 16 6
Now the question will naturally be as to what focus lens is required in our lantern, and the following tables will show how to arrive at any information that may be wanted regarding the size of picture, the focus of the lens or the distance from the screen. The usual lens fitted to lanterns is from 7 to 8 inches focus, and the following simple formulae will answer all our requirements.
To find at what distance from the screen the lantern must be placed to get a picture of desired size. Multiply the diameter of picture required by the focus of the lens and divide by the diameter of the slide.
Example. Size of picture desired 8 feet, focus of lens 7 inches, diameter of slide 3 inches.
To find what lens focus must be used when distance from the screen is known and also the size of the picture.
Multiply the distance from the lantern to the screen by the size of opening in the slide and divide by the diameter of the disc.
Example. Length of room 50 ft., diameter of slide 3 inches, size of picture 12 ft.
To find what size picture will be produced at a given distance between the lantern and screen, the focus of the lens being known.
Multiply the distance from the lantern to the screen by the size of opening in the slide and divide by the focus of the lens.
Example. Distance between lantern and screen being 40 ft., opening in slide 3 in. and focus of lens 10 in.
A distempered white wall is the best screen that can be used, but failing this an almost equally efficient substitute is a Sinclair Opaque Screen. This consists of canvas covered with white distemper, mounted on roller with pulley cords, so that it can be rolled up when not in use. Sizes up to 15 feet square can be supplied, but naturally such large sizes are not very portable, and for travelling purposes ash poles of various lengths are joined together with brass collars. Particulars of such portable stands, on on which calico and linen screens can be stretched, will be found in any catalogue.