PER 
the colours of objects are seldom seen pure 
and unmixed, but generally arrive at the 
eye broken and softened by each other ; 
and, therefore, in painting, where the natu- 
ral appearances of objects are to be describ- 
ed, all hard or sharp colouring should be 
carefully avoided. 
A painter, therefore, who would succeed 
in aerial perspective, ought carefully to 
study the effects which distance, or the dif- 
ferent 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 men- 
tioned, and repi-esent it accordingly ; so 
that, in a picture of various coloured objects, 
he may be able to give each' original colour 
its own proper diminution 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 the brightness of the colours of objects 
close to the picture, must serve as a stan- 
dard ; with respect to which, all the same 
colours, at different distances, must have 
a proportional degradation in like circum- 
stances. 
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 an- 
other, or for the general colours .employed 
in the rest of the picture, it will appear 
too glaring, seem to start out of its place, 
and throw a flatness and damp upon the 
rest of the work ; or, as the painters ex- 
press it, the brightness of that colour will 
kill the rest. 
Perspective glass, in optics, differs 
from a telescope in this : instead of the con- 
vex eye-glass placed behind the image, to 
make the rays of each pencil go parallel to 
the eye, there is placed a concave eye-glass 
as much before it; which opens the con- 
verging rays, and makes them emerge pa- 
rallel 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 perspec- 
tives, are so contrived that a person can 
PER 
view any one in a public place, as the opfera 
or play-houses, without it being possible to 
distinguish who it is he locks at. See 
Opera glass. 
Perspective plane, is the glass, or other 
transparent surface, supposed to be placed 
between the eye and the object, perpendi- 
cular to the horizon. It is sometimes called 
the section, table, or glass. 
PERSPIRATION, in medicine, the 
evacuation of the juices of the body through 
th# pores of the skin. Perspiration is dis- 
tinguished 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. Independently 
of the sensible perspiration, which is called 
sweat, and which is accidental, there is, 
moreover, one that is insensible, acting 
more or less at every instant, and which 
none could conceive to be so abundant as 
it is, before the experiments of Saneto- 
rius. This celebrated philosopher had the 
resolution to pass a part of his life in a 
balance, wherein he weighed himself, in 
order to determine the loss occasioifed by 
the effects of the insensible perspiration. 
He has found that this kind of evacuation 
causes us to lose, in the space of twenty- 
four hours, about five-eighths of the nutri- 
ment, which we. have taken. Dodard, in 
repeating afterwards the same experiments, 
has had regard to the difference of age, and 
is convinced tliat a person perspires much 
the most in his, youth. But the philosophers 
who have directed their attention to this 
object, have not sufficiently distinguished 
the effect of the perspiration or transpira- 
tion which is performed by the lungs, and 
of which the matter escapes by expiration, 
from the effect which is attributable to the 
cutaneous perspiration, or to that which 
obtains through the intermediation of the 
skin. Seguin has undertaken, in conjunc- 
tion with Lavoisier, to determine these 
two effects separately; and after having 
sought, in the usual manner, the total result 
of the transpiration, has suppressed that 
which is performed by the skin, by applying 
upon that organ a cover impermeable to 
the humour which it transmits outwardly : 
thus has been obtained the quantity of tlie 
pulmonary tfanspiration : and the mean be- 
tween the results of these experiments 
gives seven-elevenths for the ratio between 
this quantity and that of the cutaneous per- 
