Accessories. 



63 



means of an extra prism cemented to it, with a clear spot 

 in the centre, the object is examined visually in the same 

 way as usual, and when it is desired to draw on the paper, 

 the pencil-point appears to be on top of the object in the 

 field of view, and the minutest details can be exactly traced. 

 It is necessary that the paper be inclined at exactly the 

 same angle as the instrument, otherwise the image will be 

 distorted. There is another very commonly-used form of 

 camera lucida, invented by Wollaston. For this it is neces- 



FIG. 23. Abbe Camera Lucida. 



sary to put the instrument in a horizontal position, but it 

 is somewhat difficult to see the pencil-point with it when 

 working, and should be used with part of the eye over the 

 prism, while the other part views the pencil-point ; a little 

 practice soon enables this to be done. Although more 

 expensive, we do not consider it superior to the Beale's 

 neutral-tint. There are many other forms peculiar to 

 individual makers possessing more or less merit, some of 

 which may be used with the tube in any position, but 

 particulars of these can easily be gained from respective 

 catalogues. 



