PHOTOGEAPHIC PEOCESSES 249 



lantern-slides. The essentials are a good source of light, 

 and this should preferably be one of small area such as oxy- 

 hydrogen or electric arc a projection-lens which is aplanatic, 

 and a screen which has a perfectly dead- white surface, and is 

 opaque. The latter point is perhaps in some respects the 

 most important, and it is certainly the one to which least 

 attention is usually given. A small, brightly illuminated pic- 

 ture is much to be preferred to a large, badly lighted one, and 

 it therefore follows that, except for demonstrations to large 

 audiences, a picture from six to eight feet in diameter will 

 show in any ordinary lecture-theatre much better than one of 

 larger size. 



