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 ; Porte- Lumiere 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 Instru- 

 mente 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 avail- 

 able. 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 candle- 

 power 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). 



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