OPTICAL INSTRUMENTS. 189 



Daguerreotype pictures are taken on copper plates, 

 covered with a coating of silver of a sufficient thickness to 

 allow of its being finely polished. The method of proceed- 

 ing consists of five distinct operations, viz. 1. Cleaning the 

 silvered plate. 2. Rendering its surface sensitive to light 

 by exposing it to the vapor of iodine, <fec. 3. Exposing the 

 prepared sensitive plate to the focus of the camera. 4. 

 Bringing out the picture by exposing it to the vapor of mer- 

 cury. 5. Setting the picture by removing the sensitive sur- 

 face of the plate which has not been acted upon by the 

 light ; and lastly, they are sometimes colored, which is done 

 by hand in a manner similar to painting. 



Price, of camera with mahogany or rose- 

 wood box, $25.00. 

 " common stand, - $ 4.00. 

 " two coating boxes, for coating the 

 plate with iodine and quick stuff, 

 the pair, - $ 4.50. 

 " cast iron mercury bath, - $ 1.25. 

 " thermometer for mercury bath, $ 0.50. 

 " spirit lamp, 25 cts. to $ 1.00. 

 " plate vice, for holding the plate while 



cleaning, - $1.75. 



" buffing stick, for polishing the plates, $ 0.25. 

 " head rest, with arrangements for at- 

 taching to a chair, $ 3.00. 

 " box of colors and brushes, $ 3.00. 

 " plates, medium size, 2| by 3| inches, 



per dozen, $ 3.00. 



" plates, quarter size, 3f by 4 inches, 



per dozen, $ 4.50. 



Camera Lucida. (Fig. 706, next page.) By means of 

 this instrument objects are represented on a sheet of paper, 

 so that an accurate drawing may be made, even by those 

 little accustomed to the pencil. In sketching from nature 

 it is of the greatest use to the artist ; an indifferent 

 draughtsman may make an accurate drawing of the view 

 before him. Portraits may be correctly taken the size of 

 life, or in any less proportion. Paintings, prints, maps, 

 drawings of machinery, instruments, furniture, &c., may be 

 drawn in true perspective to any required scale. 



