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the art of drawing by means of the hand, the eye alone controlling and guiding the tool or instrument used for delineation. The hand guided by the eye can, however, only picture or draw what is seen from one, position at a time ; for were it otherwise, a distorted view of the object would be the result, as its appearance to the eye from one point of view would be different to that from any other. All objects are made visible to the sense of seeing by the agency of light, whether natural or artificial, for without light it would be impossible to distinguish one object from another. To the artist or draughtsman, light is a stream of matter given off by a luminous body, travelling from its source in thin straight lines or rays to the object illumined, from which it is reflected or transmitted in the same way to his eye. What is seen, or is apparent to his sense of sight, he depicts or draws on his paper. If he changes his position with respect to the illumined object, he sees it differently, and obtains a different view of it ; each such view, if correctly drawn, is known as a " perspective," and would agree with that obtained in the following manner. In the diagram (Fig. 12) let HP represent a flat surface, such as a piece of ground or a floor, exposed to sunlight, and VP a sheet of glass set up on HP, in a vertical position. At any distance to the left of YP, and parallel to it, is erected a piece of fencing OO, 1 laving its top and bottom edges parallel to HP, and its side edj?es 12 MECHANICAL AND ENGINEERING DRAWING 13 1-