88 LECTURE XI. 



figure is first modelled in a mixture of wax, turpentine, and oil ; and a 

 mould being formed on this, the ultimate cast is made either of plaster, or 

 of a composition of wax with white lead and a little oil, which serves as 

 an imitation of marble. 



We have, however, much less frequent occasion to make an exact copy 

 of a solid of any kind, than to represent its appearance by means of per- 

 spective delineation. Supposing ourselves provided with proper materials 

 for drawing, we may easily imitate, with the assistance of a correct eye, 

 and a hand well exercised, the figures and relative positions of objects 

 actually before us, by delineating them in the same form as they would 

 appear to be projected on a transparent surface placed before the eye. 

 Considering the simplicity of this process, it is almost surprising that the 

 doctrine of perspective should have been supposed to require a very serious 

 study, and that material errors should have been committed with respect 

 to it, by men whose general merit in other departments of painting is by 

 no means contemptible. But it must be confessed, that when, instead of 

 imitating objects immediately before us, the pencil is employed in embody- 

 ing imaginary forms, calculated either for beauty or for utility, a great 

 degree of care and attention may be necessary in order to produce a true 

 representation of objects, which are either absent, or have no existence : 

 and here memory and fancy only will scarcely ever be sufficient, without a 

 recurrence to mathematical principles. To architects therefore, and to 

 mechanics in general, a knowledge of perspective is almost indispensable, 

 whenever they wish to convey by a drawing an accurate idea of their 

 projected works. 



If any assistance be required for the delineation of an object actually 

 before us, it may easily be obtained in a mechanical manner, by means of 

 a frame with cross threads or wires interposed between the eye and the 

 object. The eye is applied to an aperture, which must be fixed, in order 

 to preserve the proportions of the picture ; and which must be small, in 

 order that the threads and the more distant objects may be viewed at the 

 same time with sufficient distinctness. The paper being furnished with 

 corresponding lines, we may observe in what division of the frame any 

 conspicuous point of the object appears, and may then represent its image 

 by a point similarly situated among the lines drawn on our paper ; and 

 having obtained, in this manner, a sufficient number of points, we may 

 complete the figures by the addition of proper outlines. Sometimes, for the 

 delineation of large objects requiring close inspection, it has been found 

 useful to employ two similar frames, the one a little smaller than the 

 other, and placed at a certain distance from it ; so that every part of the 

 object, when seen through the corresponding divisions of both frames, 

 appears in the same manner as if the eye were situated at a very remote 

 point. It was in this manner that the elegant anatomical figures of Albinus 

 were executed. (Plate VII. Fig. 99.) 



But if it be required to lay down, in the plane of a picture, the projection 

 of an object of which the actual dimensions and situation are given, we 

 may obtain the requisite measures from the properties of similar triangles, 

 and the consideration of the rectilinear motion of light. We may consider 



