338 OPTICAL PROJECTION 



rotated, some very peculiar effects may be produced. In 

 these experiments the light in the lantern should be care- 

 fully adjusted to its best distance from the condensers. 



Another beautiful method of projecting perforated plates or 

 cards I adapt from Prof. Dolbear's ' Art of Projecting ' with 

 the heliostat. The effect is, indeed, largely due to a strong 

 prismatic dispersion by the edges of the lens ; but the beauty 

 of the result makes the experiment worth a place. The 

 radiant is drawn back in the lantern, so that the rays from 

 the condenser cross in a focus 5 or 6 inches in front of it 

 about at the end of, or slightly within, the flange-nozzle, 

 answers very well. Adjust in the optic axis another lens 

 of about equal diameter and of deep curve, such as one of 

 the condenser-lenses themselves, at a distance which gives a 

 luminous disc 6 to 10 feet in diameter. Now hold a piece 

 of perforated card or zinc (very coarse is best) in a position 

 easily found, either in front of or behind the second lens ; the 

 result will be most gorgeous chromatic patterns. Instead of 

 a simple plate a revolving Kaleidotrope (p. 144) may be used, 

 when the effect will be magnificent if the zinc is of coarse 

 pattern. As already stated, however, it is only partially due to 

 diffraction. 



CHAPTER XXI 



LANTERN POLARISING APPARATUS 



194. The Elbow Polariscope. The simplest and cheapest 

 polariscope for lantern work consists of a bundle of thin glass 

 plates as a polariser, arranged at the back of an elbow, as in 

 fig. 185. The end N of the elbow is made to fit on the flange- 

 nozzle of the lantern, and the elbow is of courss so constructed 

 that nearly parallel light from the condensers entering at N 

 falls upon the glass plates G at the polarising angle of 56. 



