The stronger the source of light, the more contrasty the slide must be. Lantern work in the family circle or before a small audience with lights of low intensity calls for comparatively soft slides, which would seem somewhat flat if shown in a large hall with the customary high power illuminants.' copying pencil on smooth paper. This is then applied for 15-20 seconds to a glass covered with gelatine (e.g. a fixed photographic plate, which must have been washed and dried). This is dipped in water and blotted with a non-fluffy blotting paper when about to take the impres Sion. The progress of the operation is followed through the glass. The lines pass from grey to violet in penetrating the gelatine. They do not transfer to gelatine which is too dry, and they spread if it is too wet. By using a gelatine tinted pale green, excessive brightness on the screen is avoided, and at the same time the contrast of the violet is slightly increased (A. Odencrants, 192o).
782. Binding Lantern Slides. The dewing which appears on a slide during projection, and the melting of the gelatine of a slide kept for some time in the lantern, are often due to an excess of moisture retained in the gelatine or in masks and labels which have not been sufficiently dried before the slide was bound up.
Varnishing lantern slides (§ 477), after drying by heat and then cooling, is a very useful means of preventing any re-absorption of moisture by the gelatine. Failing this, it is at least necessary to dry the slides, fitted with their masks and labels, before binding them up.
A judicious limitation of the picture is quite as necessary with lantern slides as with paper prints ; it can only be effected at this stage by applying a mask to the slide.' just as prints from various negatives are trimmed to different shapes and sizes, so it is not possible to get the best effect from slides if one is limited to the few masks available commercially. Among these masks those with fancy openings are to be avoided, and discretion must be used in employ ing circular or oval openings, which are, how ever, sometimes valuable to conceal a marginal defect in a slide.
The best method is to have a quantity of gummed strips of opaque paper of various widths ranging from ± in. to 1, in. These can be stuck to the emulsion surface of the slide, encroaching very slightly on marks previously made with a For this it is necessary to choose as thin a paper as possible, so as not to acid appreciably to the thickness of the glasses by the superposition of the strips. Paper pro jecting from the glasses must be cut only after the gum is dry.
Mistakes in the position of the pictures on the screen are unavoidable on the part of the lanternist unless some distinctive mark clearly shows the correct placing of the picture on each slide. The customary conventional signs are
as follow— These marks must be placed on the mask before binding up with the cover-glass, so that the label may be protected as well as the picture.
The white spots may be used for numbering the slides.
The free spaces available on one or other side of the mask (portions not covered by the binding strips) may be used for labels showing, for instance, the name of the owner on the right hand side, and a brief description of the subject on the left. These labels must be sufficiently far from the spots to avoid all confusion. Papers of different colours may be used to label slides of different sets. Titles, etc., can also be written in white ink on the black paper.
The gummed strips and the labels having been thoroughly dried, the slide is bound-up with a thin cover-glass of the same size in order to protect the image against scratches and finger markings. As far as possible these glasses must not be thicker than ,'„ in., and must be free from scratches, bubbles, etc., at least in that portion covering the image. The surface placed in contact with the slide must be perfectly clean and dry. After placing this glass on the slide, which has been dusted, and having seen that both glasses are practically the same size, the two are bound together with gummed strips.' In binding, use may be made of either one single strip, about I in. longer than the total length of the four sides of the slide, or four separate strips, one for each side. Experience has shown that the use of four short strips enables better results to be obtained more quickly. The one objection to this plan is that it does not ensure complete sealing of the corners. This is of little importance if the slides have been varnished.
With a sponge moisten the back and then the front of the strip laid flat on a table. Place the slide and cover-glass (held together) along the middle of the strip.' Raise the sides of the strip to ensure a sufficient adherence for an instant, and turn the slide over. Then pass the finger or a small pad along the edge in order to cause the gummed paper to stick to the edges of the two glasses, and then, starting from the middle of the side, press the sides of the strip against the glasses by rubbing between the thumb and forefinger. Finally, rub towards the centre of the slide so as to stretch the gummed paper.' Any paper projecting at the two ends is then cut off after another strip has been fixed.