DEVICES FOR SEEING 



285 



so that the light entering the hole will be reflected by the mirror 

 down on to the other end which is to be the base of the instrument. 

 Tack an ample, dark curtain on to the open top of the box, 

 fastening it at the end near the drillhole and to the adjacent 

 sides so that when head and shoulders are introduced into the 

 box it will cover them and exclude the light. Set the instrument 

 base down on a table out of doors or on legs fastened to the base. 

 Lay a piece of white paper on the base inside the box. Light 

 now coming through the drillhole is reflected by the mirror 

 on to the paper, and forms there an image of the object to be 

 sketched or of the land- 

 scape to be mapped. 

 With pencil in hand and 

 your head and shoulders 

 under the curtain you 

 can trace the outline of 

 the picture desired. The 

 image will be much 

 brighter if a long-focus 

 camera lens is used in 

 place of the drillhole be- 

 cause it will admit much 

 more light (Fig. 133). 



The ray of light will 

 be bent out of the 

 straight course in which 

 it usually travels (i) when it strikes a reflecting surface like 

 that of a mirror; (2) when it enters or leaves a substance more 

 or less optically dense than the one in which it is traveling, 

 as when it enters the water from the air or passes through a 

 glass lens. We must undertake to comprehend some simple 

 laws of reflection and refraction in order to understand such 

 instruments as the magnifying glass, telescope, camera, and 

 other contrivances that man has invented in order to see better, 

 and farther, and longer. 



FIG. 133. The camera obscura 



