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 



