THE CAMERA LUCIDA. 131 



pencil may be moved along the outline 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 on 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 compli- 

 cated subject. If the operation is 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 be required for it to be completed at one 

 sitting, the paper should be fixed to the table by a weight, 

 or on a board by drawing-pins. An excellent plan to 

 adopt is to fix the microscope on a piece of deal about two 

 feet in length and one foot in breadth, and to pin the 

 paper to the same ; there will then be no risk of the shift- 

 ing of the paper, as, when the wood is moved, both micro- 

 scope and paper will move with it. In all sketches made 

 by the camera, certain things must be borne in mind ; the 

 eye, when once applied to it, should be kept steadily fixed 

 in one position ; and if the sketches are to be reserved for 

 comparison with others, the distance between the paper 

 and the camera should be always the same. A short rule 

 or a piece of wood may be placed between the paper and 

 the under-surface either of the compound body or the arm 

 supporting it, in order to regulate the distance, as the size 

 of the drawing made by the camera will depend upon the 

 distance between it and the paper. It is also very desirable, 



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