!I4 LECTURE XI. 



the ultimate cast is made either of plaster, or of a composition pf 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 an}' kind, than to represent its appearance by means of perspective 

 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 merits -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 embodying 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 suffi- 

 cient, without a recurrence to mathematical principles. To architects there- 

 fore, and to mechanics in general, a knowledge of perspective is almost in- 

 dispensable, whenever they Avish 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 

 MS, 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 suffici- 

 ■ ent distinctness. The paper being furnished with corresponding lines, we 

 may observe in what division of the frame any conspicuous point of the ob- 

 ject 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 suflicient number of points, we may complete the figures by the addition of 



