DEVICES FOR SEEING 



301 



surrounding objects so that the image is seen 

 on a dark background. If you will take two 

 convex lenses, one in each hand, and hold one 

 at arm's length as an objective, the other 

 near your eye as an eyepiece, and hold them 

 both in line with some distant object, you 

 can, by varying the distance between them, 

 get the effect of the telescope without a tube. 



In the magic lantern or stereopticon, the 

 light from some source of illumination, as an 

 electric lamp, is made to converge by convex 

 lenses on to the transparent glass slide that 

 bears the picture to be shown. The picture 

 is printed on the gelatine film on the slide 

 and must, of course, be transparent. The 

 light from the condenser goes through the slide 

 to the objective. The slide is at one of the 

 conjugate foci of this convex lens which we 

 call the objective, whose other focus is at the 

 screen. Since the slide is near the objective, 

 the screen will be far away and the image 

 formed will be much larger than the picture 

 on the slide. (See Fig. 150.) 



In the more expensive types of lenses in 

 the camera, microscope, and telescope, the 

 lens is made of several elements or separate 

 lenses that are mounted together to make the 

 so-called lens. This is necessary because of 

 two defects in any single lens: (i) spherical 

 aberration, (2) chromatic aberration. 



If you will hold in your hand any large 

 convex lens like a large reading-glass and look 

 through it toward the window, then move it 

 nearer to or farther from your eye until you see 

 the image of the window, you will note that the 



