198 SCIOPTICON MANUAL. 



the various parts of the lantern and the objects to be 

 shown properly centred has been dwelt upon at some 

 length. Now, however, the direction is to raise the light 

 about an inch above the centre of the condenser, which 

 can best be done by sliding a narrow board under the 

 blow-pipe. The maximum of illuminating power is 

 obtained in the usual way, by pushing the light back- 

 wards and forwards, and the correct focus is obtained 

 by means of the front sliding tube. Any dark portions 

 of the image may be removed by turning the kaleido- 

 scope round a very little to the right or left. 



The instrument, before using, should be warmed, to 

 prevent what is popularly known as the " steaming of 

 the glass." 



Hackwork frames, containing pieces of colored glass, 

 are supplied by the opticians; but exceedingly beautiful 

 effects are obtainable with the chromatrope, a piece of 

 perforated zinc, the bow and the wards of a key, grasses, 

 feathers, a bunch of oats, etc., etc. 



Till: OXYIIYDROGEN POLARISCOPE. 



Fig. 57 shows the Oxyhydrogen Polariscope, which 

 consists of two tubes inclined to each other at an angle 

 of 56 45', and truncated at their points of junction; 

 the oval space thus formed being closed by some ten or 

 twelve pieces of thin crown glass, the lowest of which 

 is blackened to absorb the polarized ray. This appa- 

 ratus replaces the object-glass of the lantern, which 

 should have condensers not less than 3 inches diameter. 

 When attached, it will be seen that the light emanating 

 from the point L, after passing through the condensers 

 C", becomes incident on the crown glass G, inclined at 

 the polarizing angle (56 45') ; the reflected, and in this 

 case polarized, light then passes through the selenite, 



