LIGHT AND VISION 



153 



such a lens and falls on the eye at the proper focus, the eye 

 estimates the size of the object as if the rays come straight 

 from it, and it therefore appears to be larger, or, as we say, it 

 is magnified. The double concave lens is often called a dimin- 

 ishing glass because objects viewed through it appear smaller 

 than they really are, due to the spreading of the light rays. 

 The microscope and telescope consist essentially of two sets of 

 lenses, one at each end of a light-proof tube. The set nearer 

 the object to be viewed produces a magnified image of it within 

 the tube, and this is further magnified by the lenses at the 

 opposite end. 



129. The Camera. The camera is 

 essentially a light-proof box with a suit- 

 able set of lenses for focusing on a sen- 

 sitive plate or film the light rays from an 

 object. A shutter prevents light from 

 entering the camera until the picture is 

 to be made. Then a very short exposure 

 admits sufficient light to affect the emul- 

 sion on the sensitized plate and make the 

 picture. In this picture, or negative, the 

 light and shade of nature are exactly re- 

 versed, since light objects reflect the most 

 light into the camera and produce the 

 greatest change in, or darkening of, the sensitized surface. 

 When prints are made from negatives, however, the darker 

 parts hold back the light more than others and thus the fin- 

 ished photograph reproduces the original scene in its proper 

 lights and shades. The projection lantern or stereopticon 

 is like a camera reversed. The light is sent through a 

 semi-transparent slide and enlarged by a suitable set of 

 lenses as it is projected on the screen. The eye is also much 

 like the camera. The cornea and crystalline lens focus the 

 rays of light on the retina or sensitive part at the back of the 



FIG. 55. Microscope. 



