DIFFERENT FORMS. 



of 45 to the top of the box. Over this mirror is a lid A, movable on 

 hinges, under which in the opening is set a square plate of ground 

 glass ; the lid A is provided with arrangements by which it can 

 be fixed at any desired inclination to the plate of ground glass, 

 so as to shade the latter from the light ; sides are sometimes 

 provided to exclude the lateral light ; which may also be accom- 

 plished by throwing a dark- coloured cloth over the box. 



The rays which produce the picture, entering through the lens 

 B, fall upon the mirror M, by which they are reflected upwards, 

 to the plate of ground glass N, on which they produce the picture. 

 The instrument is brought into focus by drawing out the end o of 

 the box, until the picture appears with sufficient distinctness on 

 the glass N. 



A leaf of tracing paper, being laid upon the glass, the picture 

 is seen through it, so that it can be traced with facility and 

 precision. 



7. The" form of camera usually employed for photography is 

 represented in fig. 5 ; it is more simple in its construction than 

 those already described, neither the prism nor the oblique mirror 

 being used. The convex lens, or its optical equivalent, is set in 

 a tube at one end of a square box, in which another square box 

 slides like a drawer ; in the end of this last, a plate of ground 

 glass is let in, by means of grooves, so that it can be inserted and 

 removed at pleasure ; the instrument is brought into focus, either 

 by sliding the one box within the other, or by a rack and pinion 

 in the groove. When the picture is distinctly delineated upon 

 the ground glass, the latter is drawn out, and a case containing 

 the daguerreotype-plate or photographic-paper is inserted in its 

 place. The paper or plate being, in the first instance, screened 

 from the reception of the picture by a plate of metal or board let 

 into a groove in front of it. When all is prepared for the opera- 

 tion, this screen is suddenly raised by the operator, and the 

 picture allowed to fall upon the prepared paper or plate, and 

 being allowed to continue there a certain number of seconds, more 

 or less according to the brightness of the light, the screen is again 

 suddenly let down, and the case containing the paper or plate is 

 withdrawn from the groove, and the paper or plate is submitted to 

 certain chemical processes by which the picture is brought out 

 and rendered permanent. 



The cameras which are adapted to photography require to be 

 constructed with greater attention to optical precision than those 

 which are used for other purposes in the arts. The focal length 

 of the lenses being much shorter, optical expedients must be 

 adopted for the removal of spherical aberration, which are not 

 necessary in other applications of the instrument. The nature of 



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