344 Description of the Camera Lucida. 



direction that T see my paper through the glass. I niigtit 

 then lake a sketch ol" it, but the positions of the objtcia 

 would be reversed. 



To obtain a direct view, it is necessary to have two reflec- 

 tions. The transparent glass must for tliis purpose be in- 

 clined 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 appt'ar 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 posi- 

 tions. 



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 diflerence of their distances, and the ef- 

 forts of successive adaptation of the eye to one or to the other 

 would become painful if frequently repeated. In order to 

 remedy this inconvenience, the paper and pencil may be 

 ■viewed through a convex lens of such a focus as to require 

 no more etVort than is necessary for seeing the distant ob- 

 jects distinctly. They will then appear to correspond with 

 the paper in distance as well as direction, and may be drawn 

 with facility, and with any required degree of precision. 



This arrangement of glasses will probably be best under- 

 stood from inspection of fig. J, (Plate VIII.) in which ah 

 is the transparent glass ; he, the lovver reflector ; hd, 3. con- 

 vex lens (of twelve inches focus) ; e, the position of the eye j 

 fgh, the course of the rays. 



In some cases, a difl"ercnt 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 constructicm is represented at ik, 

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

 2;lass in the frame Im, and moving in such a manner that 

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



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