158 History of' Mechanical Inventions, tyc. 



below the glass towards the paper at Q. By these lenses, a copy larger 

 or smaller than the object may be drawn. The camera lucida used by 

 Professor Amici is fitted up as in Fig. 18, which is half its real size. In 

 order to get rid of the reflection from the second surface of the glass, as 

 shown in Fig. 17, by the ray CDFC, which enters the glass, M. Amici re- 

 moves the polish from the part DM, so that no light is reflected at D." 



In order to prevent a reverse image of the objects produced by the me- 

 tallic mirror from being seen, a small plate AB of blackened copper is 

 placed as in Fig. 18, which stops the upper rays which a single reflection 

 would bring to the eye. A rectangular aperture, to which the eye is ap- 

 plied, is made in the copper, the smallest sides of the rectangle being lar- 

 ger than the pupil, and the other sufficient for seeing enough of the ob- 

 jects. 



Another construction which M. Amici thinks better than that of Fig. 

 16, is shown in Fig. 19, where RMNO is the progress of the ray from the 

 object, being reflected at M from the polished surface FG, of the metallic 

 mirror EDFG inclined 45° to BD. In this combination, the second reflec- 

 tion from the glass plate cannot be removed by grinding the face AC, but 

 this image may always be prevented from reaching the pupil, by giving a 

 proper thickness to the glass. 



In order to make the two faces of the glass perfectly parallel, in which 

 case distant objects will not appear double, M. Amici constructed a tri- 

 angular prism of glass, and having cut it in two, he united the parts ACD, 

 ADB, Fig. 19, so as to form a parallelopiped. By giving one of the prisms 

 a slight motion of rotation, a position was easily found in which the two 

 faces were parallel. 



A third species t of camera lucida has been formed by Amici, with a small 

 metallic mirror inclined to the great one, at an angle of 45°. The small 

 mirror, which is elliptical, and smaller than the pupil, is supported and in- 

 sulated by a small steel wire. The pencil is, in that case, seen by the outer 

 ring of the pupil. 



A fourth kind of camera lucida is shown in Fig. 20, where ABC is an isos- 

 celes right-angled prism of glass, having its face, BC, parallel to the metallic 

 mirror MN, with an aperture in it XY, Fig. 2, less than the pupil 

 RS ; the ray from Q, follows the progress of the dotted lines to the eye at 

 P, which sees the pencil through the opening XY, while the object is seen 

 in the circular segments at R and S. 



The last and the best construction is shown in Fig. 22, where ABC is an 

 isosceles prism of glass, whose base, AB, forms an angle of 45° with MN. The 



* We have constructed these with plates of topaz, which often split with sur- 

 faces perfectly polished, and always mathematically parallel. An inclined edge, as 

 at DE, Fig. 16, can always be got, and often a rough water-worn edge. 



•f- In place of a metallic mirror, we have found small and perfect crystals of ruby 

 silver, blende, specular iron, and oxide of tin, much better fitted for this purpose, 

 and much easier obtained. If the crystal used is extremely thin, a variation of the 

 position of the eye enables us to vary the relative illumination of the pencil of the 

 tinge— Ed. 



U 



