THE CAMERA LUCIDA. 



distance of the paper, can obtain a picture of the object on any 

 scale he may desire. 



To render this more apparent, let c, fig. 5, be the place of the 



Fig. 5. 



camera, and A B the object, whose visual angle will therefore be 

 A c B. Jf the paper be placed at P P, the lines c a and c b, drawn 

 to the extremities of the image upon it, will make the angle a c b 

 equal to A c B, so that the visual angle of the image a b, will be 

 equal to that of the object A B. 



If the paper be now removed to P' p', the visual lines c a, c &, 

 continued to it at a! &', will still be those which mark the extre- 

 mities of the image, whose visual magnitude will therefore be 

 measured by the same angle. But the space which the image 

 covers on the paper at p' P', or what is the same, the actual length 

 of the optical picture on the paper, will be greater than at P P, in 

 the proportion of a' 5' to a 6, or what is the same, to the dis- 

 tance of P' P' to that of P P from c. 



In the same manner it will appear, that if the paper be succes- 

 sively moved to greater distances, such as P" P", and r'" P"', the 

 190 



