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causes, the principal of which are the fol- 

 lowing. 



1. From the objects being removed to 

 a greater distance from the eye, whereby 

 the rays of light winch .t reflects are less 

 vivid, and the colour becomes more di- 

 luted and tinged, in some measure, by the 

 iaint bluish cast, or with the dimness or 

 haziness of the body of air through which 

 the rays pass. 



2. From the greater or less degree of 

 light with which the object is enlighten- 

 ed ,- the same original colour having a 

 different appearance in the shades from 

 what it has in the light, although at an 

 equal distance from the eye, and so in 

 proportion to the strength of the light or 

 shade. 



3. From the colour of the light itself 

 which falls upon it, whether it be from 

 the reflection of coloured light from any 

 adjacent object, or by its passage through 

 a coloured medium, which will exhibit a 

 colour compounded of the original colour 

 of the object,, and the other accidental co- 

 lours which the light brings with it. 



4. From the position of the surface of 

 the object, or of its several parts with re- 

 spect to the eye ; such parts of it appear- 

 ing more lively and distinct than those 

 which are seen obliquely. 



5. From the closeness or openness of 

 the place where the object is situated; 

 the light being much more variously di- 

 rected and reflected within a room, than 

 in the open air. 



6. Some original colours naturally re- 

 flect light in a greater proportion than 

 others, though equally exposed to the 

 same degrees of it ; whereby their degra- 

 dation at several distances will be differ- 

 ent from that of other colours which re- 

 flect less light. 



From these several causes it happens 

 that the colours of objects are seldom 

 seen pure and unmixed, but generally ar- 

 rive at the eye broken and softened by 

 each other ; and, therefore, in painting, 

 Where the natural appearances of objects 

 are to be described, all hard or sharp co- 

 louring should be carefully avoided. 



A painter, therefore, who would suc- 

 ceed in aerial perspective, ought careful- 

 ly to study the effects which distance, or 

 the different degrees or colours of light, 

 have on each particular original colour, to 

 know how its appearance or strength is 

 changed in the several circumstances 

 above mentioned, and represent it accord- 

 ingly ; so that, in a picture of various co- 

 loured objects, he may be able to give 

 eack original colour its own proper dimi- 



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nution or degradation, according to its 

 place. 



Now, as all objects in a picture are pro- 

 portioned to those placed in the front ; so 

 in aerial perspective the strength of light, 

 and tin- brightness of the colours of ob- 

 jects close to the picture, must serve a^ 

 a standard ; with respect to which, all the 

 same colours, at different distances, must 

 have a proportional degradation in like 

 circumstances. 



In order, therefore, to give any colour 

 its proper diminution in proportion to its 

 distance, it ought to be known what the 

 appearance of that colour would be, were 

 it close to the picture, regard being had 

 to that degree of light which is chosen as 

 the principal light of the picture. For if 

 any colour should be made too bright for 

 another, or for the general colours em- 

 ployed in the rest of the picture, it will 

 appear too glaring, seem to stall out of 

 its place, and throw a flatness and damp 

 upon the rest of the work ; or, as the 

 painters express it, the brightness of that 

 colour will kill the rest. 



PERSPECTIVE glass, in optics, differs 

 from a telescope in this : instead of the 

 convex eye-glass placed behind the im- 

 age, to make the rays of each pencil go 

 parallel to the eye, there is placed a con- 

 cave eye-glass as much before it ; which 

 opens the converging rays, and makes 

 them emerge parallel to the eye. The 

 quantity of objects taken in at one view 

 with this instrument does not depend 

 upon the breadth of the eye-glass, as in 

 the astronomical telescope, but upon the 

 breadth of the pupil of the eye. 



Reflecting perspective glasses, called 

 by some opera-glasses, or diagonal per- 

 spectives, are so contrived that a person 

 can view any one in a public place, as the 

 opera or play-houses, without it being 

 possible to distinguish who it is he looks 

 at. See OVER A. glass. 



PERSPECTIVE plane, is the glass, or other 

 transparent surface, supposed to be placed 

 between the eye and the object, perpen- 

 dicular to the horizon. It is sometimes 

 called the section, table, or glass. 



PERSPIRATION, in medicine, the eva- 

 cuation of the juices of the body through 

 the pores of the skin. Perspiration i.s 

 distinguished into sensible and insensible 

 See PHYSIOLOGY. 



The skin of man and of animals is pierc- 

 ed with an infinitude of pores, through 

 which, by means of the transpiration, the 

 parts of the aliments escape which do not 

 contribute to nourishment. Independent- 

 ly of the sensible perspiration, which is 



