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INTROD UCT ION.                              9 
ideas, Which the complete knowledge of the whole 
would naturally have given us, if we had confidered it 
in the other way before. 
Another advantage of confidering obje&s thus merely 
as fhells compofed of lines, is, that by thefe means we 
obtain the true and full idea of what is call'd the out- 
lines of a figure, which has been confin'd within too 
narrow limits, by taking it only from drawings on paper; 
for in the example of the fphere given above, every 
one of the imaginary circular threads has a right to be 
confider'd as an out-line of the fphere, as well as thofe 
which divide the half, that is feen, from that which is not 
feen ; and if the eye be fuppofed to move regularly round 
it, thefe threads will each of them as regularly fucceed 
one another in the office of out-lines, (in the narrow 
and limited fenfe of the word:) and the inftant any one 
of thefe threads, during this motion of the eye, comes 
into fight on one fide, its oppofite thread is loft, and 
difappears on the other. He who will thus take the 
pains of acquiring perfe& ideas of the diftances, bearings, 
and oppofitions of feveral material points and lines in 
the furfaces of even the mofi irregular figures, will gra- 
dually arrive at the knack of recalling them into his 
mind when the objecs themfelves are not before him: 
and they will be as ftrong and perfe& as thofe of the 
moft, plain and regular forms, fuch as cubes and fpheres ; 
and will be of infinite fervice to thofe who invent and 
draw from fancy, as well as enable thofe to be more 
corre& who draw from the life.