158 PRACTICAL PHOTO-MICROGRAPHY. 



one. It is simply a box to hold the condenser, object or slide, 

 and projecting lens in certain relations to each other and to 

 the radiant, and to prevent the radiant from illuminating the 

 screen directly. The screen is usually a sheet of white fabric, 

 cloth or paper, but it ought to be, especially for purposes now 

 under consideration, an opaque white surface, as plaster on a 

 smooth wall. Paper with a smooth but not shiny surface, 

 pasted to a wall or board of sufficient size, will answer every 

 purpose. The diameter of the screen should be about one- 

 third of the length of the lecture room ; there is no necessity 

 for much more, and in any case 15 to 18 feet will be the extreme 

 likely in ordinary cases to be convenient. 



An oil lamp made on scientific principles, due attention to 

 draught being the main feature, will illuminate a ten-feet 

 screen well, an eight-feet one perfectly. Number of wicks is 

 of less consequence than the position of the wicks with relation 

 to each other, and still more emphatically with relation to the 

 condenser. A lantern full of burning wicks will give less light 

 and worse confusion than one wick or three properly placed. 

 A lantern known in America as the Scovill " Sciopticon " is 

 as good as any oil lamp we know. We figure this article 

 Fig. 36 (a). 



Perhaps the condenser is the most important part of a lan- 

 tern. It should be a double one certainly, and a triple is by 

 some considered superior. A condenser which works very 

 well with a wick lamp need not be the best for the lime-light, 

 and the converse holds equally good. Still a condenser which 

 gives an even, brilliant image, free from spherical aberration, 

 with the lime light will suit well for an oil lamp with one or 

 three wicks as usually arranged. 



Now that oxygen is obtained in our countries so easily, of 

 so good quality and at such moderate prices, we think it prob- 

 able that a lecturer who has once used the lime-light would not 

 long tolerate the nuisance of an oil lamp. There is perhaps 

 no necessity for a serious nuisance in connection with oil ; and 

 whatever trouble there is will fall not on the lecturer so much 

 as on paid assistants ; none the less the inconveniences of lime- 



