This section is from the book "Modern Photography In Theory And Practice", by By Henry G. Abbott. Also available from Amazon: Modern photography in theory and practice: A hand book for the amateur.
The making of lantern slides and transparencies although a comparatively simple operation, requires care and more or less skill and experience in order to get good results. A lantern slide or transparency is really a print on glass, being a positive and presents the picture exactly as viewed in nature. They are made in two ways, by contact and by means of the camera. Slides made by contact reproduce the view shown by the negative in exactly the same size as the negative does, while those made in the camera are on a smaller scale, that is, the view shown on a larger plate is reduced in size to go on a lantern slide plate. Special plates are made for the purpose and American lantern slide plates are 3 1/4 x 4 inches in size. English lantern slide plates are made 3 1/4 x 3 1/4 inches. The glass in these plates is very thin and of the best quality and the coating or emulsion is also thin, so much so that it is sometimes extremely difficult in the dark room to distinguish the coated from the glass side of the plate. In such an event the sensitive side can readily be determined by breathing upon the plate; the breath will condense upon the glass side but will have no visible effect on the coating.
The negative from which the contact slide is to be made is placed in a printing frame with the film side up, exactly the same as in making a print. The lantern slide is then placed in position upon the negative so that the two plates rest film to film. This is of course done in the dark room by means of the red light from the dark room lantern. Care must be exercised to see that all dust is removed from the film sides of both the negative and lantern plate and that the glass side of the negative is clean, for the smallest defect is exaggerated when the slide is placed in the lantern and the enlarged view thrown upon the screen. The negative with the lantern plate on it is held up before the red light of the dark room lamp in order to place the lantern plate in position.
Move the lantern plate around until it includes that portion of the negative which you wish to show and then place the back of the printing frame in position and clamp it down. The printing frame is now turned face down on the bench or table and the ruby lamp opened. The printing frame is then held in a vertical position in front of the lamp and about eighteen inches from it for about ten seconds, the length of exposure and the distance from the light being governed by the size and quality of the light and the density of the negative. If the negative is a medium one, eighteen inches will be about the right distance from the light; if thin, remove more distant from and if dense bring it closer to the light.
Should the negative be a very thin one it will be well to place a piece of ground glass in front of the light and lengthen the exposure. With an ordinary gas burner and medium negative, from two to five seconds exposure will be ample but with an oil light the exposure will vary from ten to fifteen seconds. A few experiments will teach you what is the proper exposure for a given negative and others can be judged from it just as you use your judgment in making Bromide or Velox prints of negatives.
The all important point in making lantern slides is to have clear glass in the highlights or sky, providing the latter is not intended to show clouds. When you have made the exposure the printing frame is again turned face down on the bench and the dark room lantern closed. The back of the frame is then opened and the lantern plate removed and developed. No special developer is required and the work is performed the same as developing a plate with the exception that care must be exercised not to carry the development too far. If the negative was one having a plain white sky, then we must stop development as soon as we see that the sky is changing in the slightest degree to a dark shade, for the object is to have clear glass in the sky, when the slide comes out of the hypo bath. When the image has developed up clear and distinct and just before the sky changes, remove the plate from the developer, rinse in water and place in the hypo bath. The ordinary bath used for negatives will answer, though a diluted bath will be better. When the plate has cleared up in the fixing, wash it for thirty or forty-five minutes and put it in the rack to dry.
It will not be necessary to develop each plate as soon as printed after you have familiarized yourself with the process and the length of exposure. Five or six exposures can be made one after the other and the plates put in a box and all the development and fixing done at one time. When the plate has dried thoroughly it is ready for the finishing touches in the way of cover glass, mat and binder. The covers consist of thin crystal glasses which are sold for the purpose. The mats are frames cut from black paper with various sizes and shapes of openings, most of them being oblong with rounded corners. The lantern slide is laid on the table, film side up, the mat placed on the film, and the cover glass, previously cleaned, is then placed on the mat. The three are then held together and removed from the table and the binding strip applied to the edges so as to hold them in place. Binding strips consist of narrow slips of tough black paper, gummed on one side. The Ideal Lantern Slide Vise, shown at Fig. 47 will be found very convenient when applying these binding strips, as the lantern slide, mat and cover glass are held firmly in position, leaving both hands free to apply the binding strips. The slide is clamped between rubber discs which do not scratch the glasses and the slide is readily revolved. A mat should always be selected which will cut out that portion of the picture which is not desirable to show upon the screen and it adds a finish to the view just as a frame does to a picture.
We will now call the reader's attention to the second way of making lantern slides, that is, by the reduction method and right here it is well to state that slides made in this way are vastly superior to those made by contact. Fig. 48 illustrates the Anthony Lantern Slide Camera, which has an oscillating frame carriage for the ground glass and plate holder, which facilitates the adjustment of the picture on the plate. It will be noted that the lens is situated about the center of the bellows but this center board and lens can be used also at the end of the bellows, thereby converting it into an extra long copying camera. This camera is made for copying 4 x 5 or smaller negatives on 3 1/4 x 4 or 3 1/4 x 4 1/4 lantern slide plates. The negative is placed in one end of the camera, film side toward the lens, where it is held by means of springs and the camera placed in front of a window or light in order to focus the scene to the right size on the ground glass. When the focus is satisfactory the plate holder containing the lantern plate is placed in position, the focusing cloth is thrown over the end where the negative is held and the camera is taken out of doors to a spot where the sky can be viewed uninterruptedly. Here the camera is pointed towards the sky and an exposure made, the length of time depending on the brightness of the day and the dense-ness of the negative. The development is then proceeded with, the same as with a contact negative. If the slides are made at night the lighting can be done by gas or flash light but in that event a ground glass should be held a few inches in front of the negative in order to diffuse the light. Of course the exposure can also be made in the day time from any window where an uninterrupted view of the sky is to be had but the sun must not shine on the negative. A piece of white cardboard can be placed at an angle outside the window to cut off any trees that shade the light and to reflect the light through the negative. Should the negative be larger than will fit in the front of the camera, then we shall have to employ a little different method. We can take a starch or soap box, remove the lid, and cut an opening in the: center of the bottom of the box which will just take the large negative. The negative can be fastened in this opening by means of thumb tacks or two wooden cleats can be tacked on in which the negative can be held. The film side of the negative should face the inside of the box and the inside of the box should be next the camera front. The camera can then be brought into position until the image on the ground glass is of the right dimensions and the focusing cloth can be spread over the intervening space between the camera and box, thus shutting off all light except that which comes through the negative. Fig. 49 illustrates the Ideal Lantern Slide Camera which is a very simple arrangement, being a wooden box with lens in the center partition, kits for the reception of the negative in one end and the lantern slide in the other. The kit for the reception of the negative is reversible so that vertical or horizontal negatives can be used. The lens being a fixed focus one the negative is always in focus. This camera is built for 4 x 5 negatives only.
If any difficulty is had in focusing the negative or the ground glass and it often happens, then the following method may be resorted to: Divide the length of the negative in inches by the length of the lantern slide to the result add 1 and multiply the sum by the focal length of the lens and the result will be the distance that the lens should be from the negative. We will assume we have a 6 1/2 x 8 negative, a 31/4 x 4 lantern plate and a lens of 6 inch focus. The process will be as follows:
8 ÷ 4 = 2 + 1 = 3 x 6 = 18.
Now we have the distance from negative to lens which is 18 inches, we must know the distance from lens to lantern plate. We proceed to divide the result 18, by the first quotient or 2 and the result will be 9 ins, or the distance from the lens to the lantern slide.
If we desire clouds in the sky of our lantern slide and the negative has none we can secure them by printing the clouds in on a separate lantern slide plate and using this plate as a cover glass for the lantern slide. Great care will have to be exercised, however, to see that the two slides blend well together at the horizon line and also that we are not putting clouds which are lighted from the right into a scene which is lighted from the left and that in all other respects the clouds match the scene and above all be careful not to get the clouds upside down.