Page:The wonders of optics (1869).djvu/256

 duction of a triangular prism, in conjunction with plates of coloured glass, but the difficulty of rendering the image and the paper of the same strength is very great. The instrument is also hard to use, from the additional difficulty of always keeping the head in the same position, for the least movement from left or right is sufficient to throw the whole drawing out.

A simple camera lucida may be made out of a small piece of looking-glass, mounted at an angle of 45°, or half-way between the horizontal and the perpendicular. If this be turned towards the drawing or view to be copied, and the left eye applied to the mirror, the image of the object will be seen on the paper below, and the pencil may be guided with the right. The proper use of this simple little instrument depends in a great measure upon the focus of each eye being the same. The light falling on the paper, too, requires very careful adjusting, otherwise the brighter object will eclipse the other. It is a good plan, too, to whiten the pencil or pen used, so that it may not so easily be lost when drawing the brighter parts of the object. We have seen excellent drawings made from plants by means of a little instrument of this kind, which simply consisted of a piece of looking-glass inserted in a cork stuck in a glass bottle.