Lantern slides are now so universally used in the lecture-room that every teacher should be able to make them. Three general classes of lantern slides, as far as the technic of making them is concerned, will be described: (1) lantern slides by contact, (2) by reducing or enlarging, and (3) by copying illustrations.
1. Lantern Slides by Contact. This method is very simple. Imagine that the lantern-slide plate is a piece of printing-out paper, and proceed just as in making a print on paper. Remember that dust on the negative or plate causes spots in the print, and that spots so small as to be almost unnoticeable in an ordinary print will be greatly magnified when they appear on the screen. Brush both negative and plate very gently with a soft clean brush before making the print. If the negative is 3 1/4 * 4 1/4 inches, it can be placed in a printing frame of that size, and the lantern slide placed upon it with the two films in contact, just as in printing paper. If there is no small printing frame, use a 4 * 5, a 5 * 7, or even an 8 * 10 frame. In such cases, put in a piece of clean glass free from scratches or bubbles, and lay the negative upon it. Lantern' slides may be printed from a portion of a 4 * 5 or some larger negative by simply placing the lantern-slide plate over the desired spot. Take great care not to scratch the negative.
A much more satisfactory method is to use a box 10 inches square and 16 inches high, inside measurement. The box should be painted white inside, and the top should be a strong piece of glass; but there should be also a wooden lid, hinged at the back and covered inside with some thick, soft cloth. Inside the box, about 4 inches from the bottom, place two electric bulbs, one red and the other white. To diffuse and tone down the light, a piece of glass, ground on one side should be placed a little more than halfway between the light and the glass top. If the light is still too strong, place a piece of white paper above the ground glass. It is most convenient to have the white light operated by a switch. With a 40-watt light, exposures will vary from 1 to 30 seconds, as the negatives are very thin or very dense. With properly exposed negatives, the exposures will vary 2 to 5 seconds.
If you are printing in a printing frame with an average negative at a distance of 3 feet from a gas-mantle lamp, try an exposure of 3 seconds; if the negative is weak, shorten the exposure; if strong, lengthen it. If you always use the same light at the same distance, you should soon be able to estimate the exposures for negatives of various densities.
If a negative is uneven, the distance from the light may be increased so as to lengthen the exposure to several seconds, thus giving time to shade the weak parts, just as in case of prints on paper. If a negative is harsh and shows too much contrast, hold it closer to the light and shorten the exposure; if weak and lacking in contrast, hold it farther away and increase the time of exposure.
Be careful not to underdevelop. A lantern slide looks stronger in the developer, and even in the hypo, than it really is.
2. Reducing and Enlarging.-If a slide is to be made from a 4 * 5 or larger negative, there must be a reduction. A camera is necessary. A 3 1/4 * 4 1/4 camera is large enough. If any larger size is used, the plateholder must be "kitted" down to 3 1/4 * 4, the standard size of lantern slides in America. In using the larger cameras, mark upon the ground glass the exact size and location of the lantern-slide plate. Fasten the negative in some convenient place where the light may shine through it: diffuse daylight is good. Then arrange the camera just as in taking any ordinary picture. The board shown in Figure 29 will be just as useful in making lantern slides as in making photomicrographs. At one end of the board fasten a frame which will hold an 8 * 10 negative and also hold kits for smaller negatives (Figs. 29B and (7). The long slot in the board will allow the camera to be fastened at the proper distance. If buildings, trees, or shadows are in the way, tilt the board so as to have a clear sky for a background.
Be very careful in focusing; it is best to examine, with a pocket lens, the image on the ground glass. In general, use a rather small stop, F16 or even F32. If reducing from an average 5 * 7 negative, in good daylight, with an F16 stop, try 2 or 3 seconds. If enlarging from a negative somewhat smaller than a lantern slide, try 8 or 10 seconds. Other things being equal, the best lantern slides are made by reduction of larger negatives and the poorest by enlargement from smaller negatives.
The superiority of the larger negative is easily demonstrated. With a 5 * 7 camera, make a negative of some elaborately ornamented building, making the building just cover the plate. Then, with a 3 1/4 * 4 1/4 camera, make a similar negative, so that the building just covers the plate. Make lantern slides from both negatives. While the building, as it appears on the lantern slide, and on the screen, is of the same size in the two cases, the one from the 5 * 7 negative will show much finer detail. The same principle holds true for all kinds of plant subjects. The small cameras are easy to carry, make good views of interesting bits of scenery, on a tripod, well stopped down, will do some fairly good scientific work; but for real scientific investigation, take a 5 * 7. If you are fond of work, or can afford to have someone else carry the heavy load, take an 8 * 10.
3 Copying Illustrations.-It is often desirable to get lantern slides from photographs, maps, or pictures in books. Here, it is necessary to make a negative and then make the lantern slide from the negative. In such cases make a 3 1/4 * 4 negative and print the lantern slide by contact. A lantern-slide plate is good for such copying. The exposure will depend upon the light, the character of the print, and the amount of reduction or enlargement. Other things being equal, the exposure will always be longer in case of enlargement than in case of reduction. If an average 5 * 7 photograph is to be copied in good diffuse daylight, with an FIG stop and a lantern-slide plate, try 15 seconds.
For making a negative of a line drawing, about natural size, with a Cramer lantern slide for a plate, at F16, dull day with no shadows, try 15 seconds.
For a diagrammatic line drawing, 16*20 inches, F16, good light, Cramer lantern slide for a plate, try 10 seconds.
If one prefers to use film, Eastman Process film is very good for copying line work. With the proper exposure and correct development it gives excellent results, the lines appearing clean, sharp, distinct, with the background perfectly black in the negative. Use the developer suggested by the manufacturer. In copying maps and line drawings where dead blacks and pure whites are desired, expose fully, but do not overexpose, and develop until the image shows plainly on the back of the plate. In such cases, the usual developers will not give as good results as a developer modified to give dead blacks upon a perfectly transparent background.
Return to Methods in Plant Histology