"Now, my little friend, while the sun drinks the dew--while all the
flowers in this old garden awake and expand, and the birds fetch
their young ones' breakfast out of the Thornfield, and the early
bees do their first spell of work--I'll put a case to you, which you
must endeavour to suppose your own: but first, look at me, and tell
me you are at ease, and not fearing that I err in detaining you, or
that you err in staying."
"No, sir; I am content."
"Well then, Jane, call to aid your fancy:- suppose you were no
longer a girl well reared and disciplined, but a wild boy indulged
from childhood upwards; imagine yourself in a remote foreign land;
conceive that you there commit a capital error, no matter of what
nature or from what motives, but one whose consequences must follow
you through life and taint all your existence.
I read this as "Now try to use your imagination"
a mental image or conception: He had happy fancies of being a famous actor.
I am not the only one to think so: http://forum.wordreference.com/showthread.php?t=2605963
Also yours I believe...