,ia26.] Philosbphical Tratmciiomfor 1824, Part IL 65 



" If We consider the effect produced by our change of position 

 with reference to any other perspective drawing, we find a 

 similar permanence of apparent position of the objects repre- 

 sented with respect to ourselves, and corresponding change of 

 direction with reference to the plane of representation, or to 

 the room in which it hangs ; and we shall be able, in this case^ 

 distinctly to trace its origin in the simplest principles of per- 

 spective drawing. 



" When two objects are seen on the ground at different dis{* 

 tances from us in the same direction, one will appear, and must 

 be represented, exactly above the other. The line joining them 

 is an upright line on the plane of the picture, and represents a 

 vertical plane passing through the eye and these objects. When 

 objects that are at different elevations, are said to be in a line 

 with us, the strict meaning is, that they are so placed that a 

 vertical plane from the eye would pass throu2;h them. Now, 

 since the upright line (drawn or supposed to be drawn on the 

 plane of the picture, and representing a vertical plane) will be 

 seen upright, however far we move to one side, and will con«^ 

 tinue to represent a vertical plane, it follows that the same set 

 of objects, even in the most oblique direction in which the re- 

 presentation can be viewed, are still in the same vertical plane, 

 and consequently will seem still to be in a line with us, exactly 

 as in the front view; seeming, as we move, to turn round with 

 us, from their first direction, toward any oblique position that 

 we may choose to assume. 



" In portraits, the phsenomena of direction with reference to the 

 spectator, and corresponding change of apparent position in 

 space when he moves to either side, depend precisely 

 on the same principles. A nose drawn directly in front 

 with its central line upright, continues directed to the spectator, 

 though viewed obhquely. Or, if the right side of the nose is 

 represented, it must appear directed to the right of the spec- 

 tator in all situations; and eyes that turn in a due degree from 

 that direction towards the spectator, so as to look at him v/hen 

 viewed in front, will continue to do so when viewed obhquely. 



XIV. Further Particulars of a Case of Pneumato- Thorax, 

 By John Davy, MD.FRS. 



A brief abstract of this communication will be found in the 

 Anmds for May 1824, p. 383. 



XV. On the Action of Jinehj divided Platinum on Gaseous 

 Mixtures, S^c* By Dr. Henry. 



Given at large in our last number. 



(To be con Untied) '" • 



New Series, vol. x. f 



