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9 The plates made for lantern slides are not nearly so sensitive. To use a technical term, they are "slower"; therefore, ruby glass is not necessary, although it is the safest. There is a glass called " dark amber," which serves admirably when backed with a piece of ground-glass. It yields a soft brownish yellow light, which does not try the eyes, and can be used without danger while working with the slow lantern-slide plates. Sunlight or daylight is the most actinic light that we have, next to which is the electric arc light, which is now being substituted for day- light in many of the best studios. Next in order is that produced by burning magnesium. These are all especially rich in blue and violet light or actinic rays. The incandescent electric light, gas jets and kerosene oil lamps yield light which also con- tains these actinic rays, but in a lesser degree. The difference may be detected by using the triangular prism. The spectrum of these is especially rich in the yellow portion, therefore the light from these sources is not so actinic, but contains enough of the actinic rays to act instantly upon the sensitive dry plate. In fact, an ordinary gas jet is the source of actinic light which the author uses for exposing the slides in order to produce the image upon the plate. As " ruby" or " amber" glass stops the actinic rays of sunlight, they will also stop those in other sources of light and allow the red light (non-actinic) to pass through so that the opera-