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66 SCIOPTICON MANUAL. scapes (except when clouds are present), perfectly clear glass. The particular tone requisite to suit the positive, is a matter of taste. A warm sepia will be found suit- able for most transparencies; but each operator must exercise his own peculiar feeling in this matter. In making positives to be exhibited by the magic lan- tern, it is well to consider the variety of light to be used in projecting the picture upon the screen. Where pow- erful illumination, such as the oxy-hydrogen or magne- sium lights are used, positives may be made slightly stronger, showing more contrast than where a weaker form of illumination is employed. The slides should be protected from scratches and dust, by a piece of clear glass of the same size, neatly pasted on the edges with muslin. Positives on glass can also be made by the wet pro- cess, from negatives of the proper size, by pasting a thin strip of cardboard upon two edges of the negative (col- lodion side). The sensitive plate is prepared as usual, and is placed, while in the dark-room, in close contact with the negative, separated only by the cardboard. It is then exposed behind the negative, to diffused sunlight or artificial light, for a few seconds, returned to the dark- room, and developed. This plan admits of no change in the size of 'the negative. Mr. L. J. Marey's appara- tus for printing wet plates by lamp-light, has given sat- isfaction to many who have not an opportunity of mak- ing experiments by daylight. The proper size for glass pictures to be used in lan- terns of convenient proportions, is a debatable subject. Glasses of 3J x 3J being generally used, but advantages are claimed for a slide 3J x 4J, that have some weight. In placing this slide in the lantern, the additional length of the glass allows the corners to be held by the thumb