THE CAMERA LUCIDA. 



It will be easy from these considerations to appreciate the difficulty 

 of using this instrument, and the necessity for practice and 

 patience from those who expect to acquire facility and expertness 

 in its management. 



6. The inversion of the object produced by the reflector M M', 

 being inconvenient, a modification of the instrument was con- 

 trived, which gives an erect image ; this is accomplished by the 

 easy and obvious expedient of subjecting the rays proceeding 

 from the object to two successive reflections, the first of which, 

 as described above, would give an inverted image, which being 

 itself inverted by the second, gives an erect image of the 

 object. 



This is effected by two plane reflecting surfaces 31 21' and M' M", 

 fig. 2, placed at an angle with each other of 135 ; the one M M' 

 being inclined at 22|, with a horizontal line, and the other at the 

 same angle with the vertical line. A ray A B, coming horizontally 

 from the object, will fall upon M M' at an angle of 22V, and being 

 reflected at the same angle, will fall upon M' 3i", still at the same 

 angle, being reflected from it, in the vertical direction, c D. An 

 object A, after the second reflection, will therefore be seen erect 

 upon a level surface, before a draughtsman who stands with his 



face towards A, and stooping over the reflector M' H", sees the 

 image of A in it. 



In some forms of the instrument, the reflections are made by 

 a prism, on the principle explained in "Optical Images," (24.) 

 Thus if one reflection only be used, a rectangular prism is applied, 

 183 



