CA^IERA LUCIDA. 233 



The cap of the eye-piece having, been removed, the camera is 

 to be slid on in its stead ; if the prism be properly adjusted, a 

 circle of white hght, with the object within it, will be seen on 

 a piece of white paper placed on the table immediately under 

 the camera, when the eye of the observer is placed over the 

 uncovered edge of the prism, and its axis directed towards 

 the paper on the table. Should, however, the field of view 

 be only in part illuminated, the prism must either be turned 

 round on the eye-piece, or be revolved on its axis, by the 

 screws affixed to its frame-work, until the entire field is illu- 

 minated. The next step is to procure a hard, sharp^pointed 

 pencil, which, in order to be well seen, may be blackened 

 with ink round the point, the observer is then to bring his 

 eye so near the edge of the prism, that he may be able to see 

 on the paper, at one and the same time, the pencil point 

 and the image of the object; when he has accomplished this, 

 the pencil may be moved along the outhne of the image so as 

 to trace it on the paper; however easy this may appear in 

 description, it will be found very difficult in practice, and 

 the observer must not be foiled in his first attempts, but must 

 persevere until he accomplishes his purpose. Sometimes he 

 will find that he can see the pencil point, and all at once it 

 disappears; this happens from the movement of the axis of 

 the eye ; the plan then is to keep the pencil upon the paper, 

 and to move about the eye until the pencil is again seen, 

 when the eye is to be kept steadfastly fixed in the same 

 position until the entire outline is traced. It will be found 

 the best plan for the beginner to employ, at first, an inch 

 object-glass, and some object, such as a piece of moss, that 

 has a well-defined outline, and to make many tracings, and 

 examine how nearly they agree with each other, and, when 

 he has succeeded to his liking, he may then take a more 

 complicated subject. If the operation be conducted by lamp- 

 light, it will be found very advantageous not to illuminate 

 the object too much, but rather to illuminate the paper on 

 which the sketch is to be made, either by means of the lamp 

 with the condensing lens, or a small taper placed near it. 

 When the object is so complicated that too much time would 



