Notices respecting New Books. 383 
herein we suffer fwo sensations of colours with a negation of all 
figure, or line, between them ; and here, therefore, we are, by a 
new result, more vividly (though not more certainly) convinced 
that itis not cofour, but contrast that is the creative principle of 
any perceived visible figure, or line. 
“ ‘fo conclude. Visible figure is a positive thing to our view, 
but only a relative thing in regard to the two sensations of co- 
lours which combine to give it being: it is nothing but the local 
or co-exiended relation of one sensation to the other.—To say, 
therefore, that we perceive visible figure, is to say that we per- 
ceive the co-loeal or ¢o-extended relation which one sensation of 
colour bears to another one, felt at the same time. 
“[t follows, upon the highest kind of evidence, that visible 
figure is nothing but a creature of the percipient,—a thought of 
the mind,—yet, athought resulting from the action of some ex- 
ternal cause stimulating our visive constitution, 
“ THE LAWS OF VISION ARE MATHEMATICAL AXIOMS, 
*< The four general facts of vision are herein called only laws, 
because their subjects are, in the first place, sensible or natural 
phenomena. But it wust be insisted upon that they possess a far 
higher title, in being mathematical axioms. 
‘© What renders this consideration mest important, is, that even 
could it be proved that visible lines are not mathematical as to 
the property of being void of breadth, this (as has been already 
remarked) would not hinder the laws of vision from being ma- 
thematical axioms in the class of their evidence, the self-evident 
necessity of their truth. 
« Physical laws (it is agreed) are not necessary, in our con- 
ception: they rule what is, but may not rule what shall be: 
light may fail to excite sensations of colours in the human mind; 
and sensations of colours may, for aught we know, be excited in 
minds without eyes: all this is conceivably possible. But, to 
conceive any one sensation of colour with a boundary or line to 
it; or, to conceive any ¢wo sensations of colours at once without 
a line between them, is an impossibility of the very same class, 
as to conceive an infinite surface with a limit, or éwo contiguous 
mathematical surfaces without the line that makes them ¢wo. 
_ & Now this perceived necessity of the laws of vision, is a pa- 
ramount test that a visible line is not an external thing; because, 
it is not merely an object of sense, but is also an object of in- 
tuition;—it is not merely a thing that now is, but a thing that 
ever must be, if its co-efficients exist. Every external object is a 
thing that may not be. at any future time; and, while it exists, 
we. know not its co-efficients: but, we absolutely know the co- 
efficients of a visible line, by the saine process of rationality, 
ae and 
