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.
Wide-Angle Lens. Attach this lens in the usual way, by slipping over your regular lens, and focus as you ordinarily would, always using the large stop. It is necessary, however, to use a considerably smaller stop when making the exposure. This wide-angle lens attachment will be found most useful for confined situations, more especially interior views where the rooms are small and you desire to show as much as possible of the room.
Telephoto Lens. This attachment is to be used for making distant pictures. It will increase the focal length of the regular lens and magnify the picture so that objects at a distance can be photographed larger than with an ordinary lens. Focus in the usual manner, after the lens is attached. Always focus with a large stop, but use a considerably smaller one when making the exposure.
Ray Filter. The function of the Ideal Ray Filter is to render the different colors, as seen by the eye, in their correct values on the finished pictures, and to accomplish this a filter is employed which absorbs the violet and blue rays of light, which act most rapidly on the sensitive plate. By this we do not mean that green will appear green or yellow appear yellow, but that the green will appear a darker shade than the yellow on the picture. A light blue will ordinarily photograph white. With the Ideal Ray Filter, however, it will have a grayish tint and the whites will, of course, be white. Thus, fleecy white clouds in a blue sky will be visible in the negative when using a ray filter. Without the use of the filter, however, the sky would be totally white.
613. The advantages of a ray filter are well known, but it must be made optically and spectroscopically perfect to be of any value.
614. The ray filter should be used in photographing landscapes, flowers, colored pictures and views with mountains in the distance. After focusing your view, slip the ray filter over the front of the lens in the regular way, but bear in mind that the exposure must be increased from six to eight times when the ordinary dry plates are being used. With isochromatic plates the exposure should only be increased about three times.
Duplicator. By attaching this instrument to the lens of any camera, a person or persons can be photographed in two positions on the plate, without showing any division line where the exposures meet. We will suppose, for example, that it is desired to photograph a person sitting at either side of a table, possibly playing a game of chess or checkers, with himself for an opponent. Place the camera so that the center of the table will be in the center of the picture.
616. After the subject is seated and the camera properly focused, attach the Duplicator by slipping it over the hood of the lens, with the opening on the same side on which the subject is sitting. If the subject is sitting on the right side of the camera, you will notice that the ground-glass on the opposite side is in darkness. Use a small diaphragm for this work, about f/16. If your diaphragms are not numbered, use the second or third from the largest stop. The right half of the ground-glass, which shows the subject, should project just a little beyond the center of the plate. You are now ready for the exposure. Expose about three times as long as for an ordinary exposure. Next, without moving the camera, turn the duplicator around so that the opening will be on the other half of the lens and have the subject change over to the other side of the table. Make another exposure, giving the same time as in the first exposure.
617. Close the shutter after the first exposure, but it is not necessary to return the slide to the holder. Also see that the side of the opening in the Duplicator is perfectly vertical. Exactly the same exposure must be given both sections, and when developing, the plate will develop evenly all over, resulting in a perfect negative showing the subject in two positions, as though it were made with a single exposure. Care must be exercised that neither the camera nor the table is moved when the subject is changing from one side to the other.