CH. VIII] PREPARATION OF LANTERN SLIDES 217 



mark on it so that the operator can put it into the lantern correctly 

 without closely inspecting each slide. 



Unfortunately there is no general system of marking slides. The 

 method recommended by the British Photographic Club 

 (Bayley, p. 78) is to put two white spots on the upper edge of the 

 slide (fig. 113). Two spots are necessary for the square slides, but 

 for oblong slides one "spot" or mark is sufficient (fig. 112). 



In America it is common to have the mark or spot on the lower 

 left hand corner of the slide ( 112), then when the slides are in a 

 pile for inserting in the lantern the spot will be turned upward 

 (fig. 8) as it must be to give an erect screen image. In the British 

 method of "spotting" the slides would have the spots on the lower 

 edge when piled up ready for insertion in the lantern. 



336. Coloring lantern slides. Photographic lantern slides 

 have been colored from their first production. To do this in the 

 best manner possible requires considerable practise and natural 

 artistic ability, but any one can color lantern slides sufficiently well 

 to add to clearness in teaching for example, veins blue, arteries 

 red, etc. All that is needed is a small artist's brush and some of 

 the desired color. 



Transparent colors in sets are on the market (see Appendix), or 

 one can employ the aqueous stains used in histology. It takes 

 some experience to get the right dilution of the color and to put it on 

 neatly with the brush. The slide should be held over some white 

 paper in a light place so that it is possible to see exactly what is being 

 done. The frame for holding slides is a convenience (fig. 113). 



If one wishes to become expert it will be a great help to study the 

 works of reference given at the head of this chapter, for they give 

 many valuable hints. 



One very important thing for the beginner to do is to test every 

 slide that is colored in the lantern to make sure that the colors look 

 right in the screen image. Sometimes a slide that looks well to the 

 naked eye in daylight will not look well when projected on the 

 screen. It is, of course, the screen image that must be satisfactory. 



The early lantern slides were mostly colored with transparent 

 oil colors, and then when entirely dry, the slide was mounted in 

 Canada balsam, and a cover-glass put on exactly as microscopic 



