THE CAMERA OBSCURA. 



length, and at right angles to its axis ; a small picture will be 

 seen upon the screen, representing the view facing the window 

 to which the axis of the lens is directed ; this picture will be 

 delineated in its proper colours, and all moving objects, such as 

 carriages or pedestrians, the smoke from the chimneys, and the 

 clouds upon the sky, will be seen moving upon it with their proper 

 motions. The picture, however, will be inverted, both vertically 

 and laterally, the sky being below and the ground above ; trees 

 and buildings will have their tops downwards, vehicles will move 

 with their wheels, and pedestrians with their feet, upwards, 

 objects on the right of the landscape will be on the left of the 

 picture, and vice versa, and all motions will be reversed in direc- 

 tion ; objects moving to the left appearing to move to the right, 

 and those which fall, appearing to rise. 



2. This remarkable optical phenomenon was discovered in about 

 the middle of the sixteenth century, by Baptista-Porta, a Nea- 

 politan philosopher, and it was not long before it assumed a 

 variety of forms, more or less useful ; the name Camera- Obscura 

 was given to it from the circumstances explained above. 



3. A great variety of forms have been given to this instrument, 

 varying according to the circumstances under which it is applied. 

 One of the most simple of these is shown in fig. 1. 



The lens, L, is inserted in an opening in the top of a rectangular 

 box, the height of which is made to correspond nearly with its 

 focal length ; the bottom of the box 

 is placed at a convenient height, to 

 serve the purpose of a desk or table 

 for the draughtsman ; a sheet of 

 drawing paper being placed upon it, 

 will receive the optical picture of such 

 distant objects as may be found in 

 the direction of the axis of the lens. 

 The lens is set in a tube, which 

 slides in the opening made in the 

 box, so that by moving it more or 

 less upwards or downwards, the in- 

 strument may be brought into focus, 

 and a distinct picture produced upon 

 the paper. An opening is made in 

 the box, at that side of it towards 

 which the bottom of the picture is 

 turned ; the draughtsman introducing through this opening the 

 upper part of his person, lets fall over him a curtain, suspended 

 from the upper edge of the opening, so as to exclude all 

 light from the box, save that which proceeds from the lens at the 

 204 



Fig. 1. 



