246 OPTICAL PROJECTION 



must be chosen so that the under surface may form a lens of 

 suitable focus to act as its own condenser, and give proper con- 

 vergence to the cone of rays. On this the excellence of the 

 projection will chiefly depend, as will be understood from pre- 

 ceding chapters ; any further 

 remarks it is to be hoped will 

 be unnecessary, except that a 

 sheet of blackened card should 

 be interposed between the 

 apparatus and the screen, so 

 as to protect the latter from 

 all scattered light. This last 

 is the only objection to the 

 arrangement, but is not great 

 if the condenser beam, mirror, 

 c., are carefully adjusted. If 

 however it be at hand, pro- 

 jections of this kind are better 

 shown, and more quickly 

 adjusted, with a vertical at- 

 tachment, in cells resembling 

 fig. 117. 



Any demonstrator will be 

 able from these hints to 

 elaborate any number of ex- 

 periments according to his 

 own requirements. The chief 

 point is to ascertain by trial 

 on the screen the most effective 

 strength for his various solu- 

 tions. The phenomena of crystallisation, &c., belong to a 

 previous chapter, and those of electro-decomposition will be 

 touched upon later. 



133. Photography. The formation, development, and 

 fixing of a photographic image is rather an attractive lantern 



. 129 Vertical Projection 



