PROJECTIONS. 25 



be seen from the window to the screen by the light re- 

 flected from the dust particles, which are always in the 

 air. Now fasten in the retort-stand a lens o four or five 

 inches in diameter, and with a focus of a foot or more, 

 and place it two or three feet from the opening, in the 

 path of the beam, and perpendicular to it. It will at 

 once be noticed that the light is converged by the lens, 

 the rays crossing each other in front of it, at its focus, 

 from which they diverge, and appear upon the screen as 

 a large disk of light. If some object, as d, (Fig. 17), 

 be placed between the opening and the lens 0, a place 

 may be found by trial, when the image of the object 

 will be seen upon the screen. The outline should be 

 well defined ; it will be inverted and much enlarged. 



Finding the right adjustment of the object and the 

 lens, so that the image is in its proper place, and has a 

 sharp outline, is called focusing. In general it is best 

 done by fixing the object in the path of the beam first, 

 and then placing the lens rather close to it, and slowly 

 moving the lens toward the screen, being careful to keep 

 it perpendicular to the beam until the image is plainest. 

 It will be well for a beginner to take a number of ob- 

 jects : some opaque, like the finger, a pencil, or a key, 

 and some transparent, as a grasshopper's wing, or a 

 piece of glass with a design drawn upon it, or a regu- 

 lar lantern transparency. A lens thus used to project 

 a picture is called an objective. 



These two pieces of apparatus, the porte lumiere and 

 the single lens, have a much wider application than one 

 unfamiliar with them might suppose. Every picture 

 made for the magic lantern, or the stereopticon, can be 

 shown with these in the day-time, even better than with 

 the others at night. Every school in the land may have 

 one, for the carpenter can make the porte lumiere^ and 



