Galilei, Galileo Dialogues on two world systems 1661, tr. Salusbury, Thomas
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305
the said irradiation of four inches to a circle that hath but two in­
ches of diameter onely, the diameter of the irradiation or Gar­
land would be ten inches, and the superficial content of the circle
would be to the area of the naked body, as 100. to 4. for those
are the squares of 10. and of 2. the agrandizement would there­
fore be 25. times so much; and lastly, the four inches of hair or
fringe, added to a small circle of an inch in diameter, the same
would be increased 81. times; and so continually the augmenta­
tions are made with a proportion greater and greater, according
as the real objects that increase, are lesser and lesser.

Superficial fi­
gures encreasing
proportion double to
their lines.

SAGR. The doubt which puzzled Simplicius never troubled
me, but certain other things indeed there are, of which I desire
a more distinct understanding; and in particular, I would know up­
on what ground you affirm that the said agrandizement is alwayes
equal in all visible objects.

Objects the more
vigorous they are
in light, the more
they do seem to in­
crease.

SALV. I have already declared the same in part, when I said,
that onely lucid objects so increased, and not the obscure; now I
adde what remaines, that of the resplendent objects those that are
of a more bright light, make the reflection greater and more re­
splendent upon our pupil; whereupon they seem to augment
much more than the lesse lucid: and that I may no more inlarge
my self upon this particular, come we to that which the true Mi­
stris of Astronomy, Experience, teacheth us. Let us this evening,
when the air is very obscure, observe the star of Jupiter; we
shall see it very glittering, and very great; let us afterwards look


through a tube, or else through a small trunk, which clutching the
hand close, and accosting it to the eye, we lean between the palm
of the hands and the fingers, or else by an hole made with a small
needle in a paper; and we shall see the said star divested of its
beams, but so small, that we shall judge it lesse, even than a sixti­
eth part of its great glittering light seen with the eye at liberty:
we may afterwards behold the Dog-stars beautiful and bigger than

any of the other fixed stars, which seemeth to the bare eye no
great matter lesse than Jupiter; but taking from it, as before, the
irradiation, its Discus will shew so little, that it will not be
thought the twentieth part of that of Jupiter, nay, he that hath not
very good eyes, will very hardly discern it; from whence it may
be rationally inferred, that the said star, as having a much more
lively light than Jupiter, maketh its irradiation greater than Jupi­
ter doth his. In the next place, as to the irradiation of the Sun
and Moon, it is as nothing, by means of their magnitude, which

possesseth of it self alone so great a space in our eye, that it lea­
veth no place for the adventitious rayes; so that their faces seem
close clipt, and terminate. We may assure our selves of the same
truth by another experiment which I have often made triall of;