This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
865. A lantern-slide is a positive transparency on a glass 3 1/4 x 4 inches square. The picture itself is smaller, and may be any shape preferred by the maker, and any size up to 2 3/4 inches each way. The edges of the picture are defined by an opening of the desired size, cut in a piece of opaque paper. This paper is laid on the slide, and masks out the edges and parts not wanted in the picture. Over the paper mask a piece of clear glass is laid, of the same size as the transparency, and the two glasses are firmly bound together by pasting a strip of paper around the edges. This covered glass serves the double purpose of holding the mask in position, and protecting the film from injury. Lantern-slide making is nothing more or less than printing on glass instead of paper. In its simplest form it is not a particle more difficult than printing on a developing paper. The lantern-slicle is, therefore, simply a positive picture on glass.
Making A Lantern-Slide. Lantern-slides may be printed from a negative either by contact or by reduction. When printing by contact, the negative is laid in an ordinary printing-frame, film side up, and upon this side is placed a lantern-plate, film side down, thus bringing both plates film to film. The back is placed in the printing-frame and a short exposure given to artificial light. In making a slide by reduction, the negative is placed before a window and is focused with a camera. A lantern plate is then placed in the plate-holder, and an exposure made on the plate. The contact method is used for making slides from small negatives. The reduction method is generally a necessity, as almost all negatives are larger than the lantern-slide, and then the latter method must be employed.