384 



OPTICAL PROJECTION 



j, fig. 209. This is filled with the fluid holding small particles 

 in suspension, which may at a pinch be prepared by adding 

 a trace of soap, or even of milk, though both of these give too 

 great coarseness of particles for fine effect. A few drops of 

 a solution of five grains gum mastic, or common resin, in one 

 ounce of alcohol, stirred into the water, answers excellently, 

 as will a few drops of French polish diluted in alcohol; but I 

 have found the finest blue from stirring a teaspoonful of the 



solution of coal-tar 

 in alcohol sold as 

 Liquor Carbonis 

 Detergens, into hot 

 water. The jar 

 should be clean, 

 and the emulsion 

 filtered into it, to 

 avoid as much as 

 possible ordinary 

 reflection from dust 

 particles. Lord 

 Bayleigh often uses 

 a mixture of ex- 

 tremely dilute so- 

 dium thiosulphate 

 with extremely di- 

 tto. 209 lute hydrochloric 

 acid, a mere trace 



of each being sufficient. The advantage and disadvantage of 

 this method is, that the effect is progressive, and finally passes 

 beyond the stage of polarisation. However, the jar being filled 

 with the fluid, the plane reflector K is arranged over it at an 

 angle of 45, so as to throw the light from the polariser N (here 

 shown as the Nicol analyser) down through the jar. Behind 

 are arranged two reflectors M M of plain looking-glass, enclos- 

 ing the jar within an angle of about 100, which give by reflec- 



