CAMERA LUCIDA. 



two reflections 

 give the natural 

 position, 



The objects 

 and the paper 

 cannot be dis- 

 tinctly seen at 

 once. 



Arrangement 

 of the glasses. 



Another con- 

 struction. 



Difference in 

 the latter in- 



flections. The transparent glass must for this purpose be 

 inclined to the perpendicular line of sight only the half of 

 45°, that it may reflect the view a second time from a piece 

 of looking glass placed beneath it, and inclined upwards at 

 an equal angle. The objects now appear as if seen through 

 the paper in the same place as before ; but they are direct 

 instead of being inverted, and they may be discerned in this 

 manner sufficiently well for determining the principal 

 positions. 



The pencil, however, and any object, which it is to 

 trace, cannot both be seen distinctly in the same state of 

 the eye, on account of the difference of their distances, 

 and the efforts of successive adaption of the eye to one or 

 to the other, would become painful if frequently repeated. 

 In order to remedy this inconvenience, the paper and pen. 

 cil may be viewed through a convex lens of such a focus, 

 as to require no more effort than is necessary for seeing 

 the distant objects distinctly. These will then ap- 

 pear to correspond with the paper in distance as Avell a.s 

 direction, and may be drawn with facility, and with any 

 desired degree of precision. 



This arrangement of glasses will probably be best under- 

 stood from inspection of Fig. I. a b in the transparent glass ; 

 b c the lower reflector ; b d a convex lens (of twelve inches 

 inches focus) e the position of the eye ; and^/" g h e the 

 course of the rays. See PI. I. 



In some cases a different construction will be preferable. 

 Those eyes, which without assistance are adapted to seeing 

 near objects alone, will not admit the use of a convex glass ; 

 but will on the contrary require one that is concave to be 

 placed in front, to render the distant objects distinct. The 

 frame for a glass of this construction is represented at i £, 

 (fig 3.) turning upon the same hinge at h with a convex 

 glass in the frame 1 m, and moving in such a manner, that 

 either of the glasses may be turned alone into its place, as 

 may be necessary to suit an eye that is long or short sighted. 

 Those persons, however, whose sight is nearly perfect, 

 may at pleasure use either of the glasses. 



The instrument represented in that figure differs moreover 



in other respects from the foregoing, which I have chosen 



5 ' to 



