Optic projection : principles, installation and use of the magic lantern, projection microscope, reflecting lantern, moving picture machine, fully illustrated with plates and with over 400 text-figures (1914)

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CHAPTER VI THE MAGIC LANTERN WITH SUNLIGHT: HELIOSTATS § 230. Apparatus and material for Chapter VI: Suitable room for projection, preferably one with southern exposure ; Screen of proper size ; PorteLumiere or hand-regulated heliostat; Heliostat with clock-work for regulation; Condenser for bringing the parallel rays of sunlight to a focus (plano-convex or achromatic combination) ; Slide-carrier and projection objective. See also Ch. I, § i. § 231. Historical. For the history of the magic lantern and all other projection apparatus with sunlight, see the Appendix. For Foucault's clock-driven heliostat see his: Recueil des Travaux Scientifiques, 1878, pp. 427-433. For the Heliostat of Mayer, using a lens and prisms, see Amer. Journal of Science, IV Ser. Vol. IV, (1897), pp. 306-308. For the Heliostats of Fuess, see C. Leiss, Die Optischen Instrumente der Firma R. Fuess, 1899, pp. 284-305. For Heliostats like fig. 82, see Ambronn's Handbuch Astron. Instr. p. 649, fig. 637. Dolbear. — Art of Projecting. LIGHT FROM THE SUN § 232. The limitless supply of light from the sun would be used in preference to any artificial source if it were only always available. In many regions it is available during most of the year, and will no doubt be much more utilized as time goes on. Its use is strongly recommended in sunny regions. The sun is the brightest known source of light. Its intrinsic brilliancy is, in round numbers, 421,000 candle-power per square centimeter (2,720,000 candle-power per sq. inch). (See § 23 2a). Sttnlight also serves as the standard for color values. § 232a. The intrinsic brilliancy of the sun. — The intrinsic brilliancy of a source can be determined if its area and its candle-power are known. With the sun it is in convenient to make the reckoning in these terms as both the candlepower and distance are so enormous. The light from the sun near the zenith in clear weather amounts to 288,000 meter candles, that is, the sunlight is as powerful as the illumination due to 288,000 standard candles at a distance of one meter. (A. Arrhenius — Lehrbuch der kosmischen Physik). 138