362 Notices respecting New Bookse 
felt at the same time, they must meet by their nearest edges, and 
this meefing we must perceive as a line, ~ 
“ Inst. If the eye traverse either the firmament, or the ocean, 
until it arrive at, and take in, amy second colour ; the evidence we 
have for this fact can be no other than our being conscious where 
one sensation of colour ends, because thé other begins. This 
meeting of the two sensations of colours, is @ line of contrast 
and of contiguity in our view: and a pereeived line, therefore; 
is purely nothing but @ thought of discrimination, which we 
make between two of our owri sensations. At the same time it 
is plain, that we can no more avoid perceiving the contrast, and 
the extended direction of this contrast, than we can avoid being 
conscious of the two different sensations of colours which form 
this contrast. 
: “ Third Law:—Formative. 
« Prop. When any two unblended sensations of colours are felt 
at the same time, and are so disposed as that one of them em- 
braces or surrounds the other, we must perceive a line of junc- 
tion, which is where the embraced sensation meets that which 
embraces it. Such a line must return into itself, and thus is 
formed every complete figure that the visive faculty can strictly 
apprehend. 
<¢ Inst. When we look at the moon, surrounded by the azure 
sky, we suffer a sensation. of silver white, embraced by a sensu 
tion of azure, and the line perceived between these two sensa- 
tions returns circularly into itself; which people take for the 
circle of the moon. 
<* It must be an obvious truth (although it is overlooked by 
Proclus) that, whatever be the hues or tints of the two sensations 
employed, there cau be but one universal principle that gives any 
perception of a line between them; and this principle is a per- 
ception of contrast. 
“ Fourth Law,—Unformative. 
‘¢ Prop. When any two sensations of colours are felt at ence, 
and ase blended or softened at their nearest edges, they never 
can be perceived as forming any dime between them, not, even, 
if their distant parts be of the most opposite colours. 
<¢ Inst, Let any surface be conceived to be black all round its 
edge, and white in its centre, and let the two colours run gra- 
dually into each other: no line can ever be perceived from look- 
ing within the field of this surface, 
- “Tnnumerable other instances of this fact may be had, as when 
we look at waving corn, or shot silks, spheres, mirrors, or drink- 
ing-glasses. ’ u 
“ This fourtlrlaw strikingly illustrates the other three; because 
~ herein 
