1angle in the Starry Sphere, that inſiſteth upon the diameter of
the Star, and by the proportion of the ropes thickneſſe to the
diſtance from the eye to the rope, by the table of Arches and
Chords, I have immediately found the quantity of the angle;
ſing all the while the wonted caution that is obſerved in taking
angles ſo acute, not to forme the concourſe of the viſive rayes
in the centre of the eye, where they are onely refracted, but
beyond the eye, where really the pupils greatneſſe maketh them
to concur.
the Star, and by the proportion of the ropes thickneſſe to the
diſtance from the eye to the rope, by the table of Arches and
Chords, I have immediately found the quantity of the angle;
ſing all the while the wonted caution that is obſerved in taking
angles ſo acute, not to forme the concourſe of the viſive rayes
in the centre of the eye, where they are onely refracted, but
beyond the eye, where really the pupils greatneſſe maketh them
to concur.
A way to
ſure the apparent
diameter of a ſtar.
ſure the apparent
diameter of a ſtar.
* Rendred in
Latine Corum, that
is to ſay,
weſt.
Latine Corum, that
is to ſay,
weſt.
* i.e. Is
ded by.
ded by.
SAGR. I apprehend this your cautelous procedure, albeit I
have a kind of hæſitancy touching the ſame, but that which moſt
puzzleth me is, that in this operation, if it be made in the dark
of night, methinks that you meaſure the diameter of the
ted Diſcus, and not the true and naked face of the Star.
have a kind of hæſitancy touching the ſame, but that which moſt
puzzleth me is, that in this operation, if it be made in the dark
of night, methinks that you meaſure the diameter of the
ted Diſcus, and not the true and naked face of the Star.
SALV. Not ſo, Sir, for the rope in covering the naked body
of the Star, taketh away the rayes, which belong not to it, but
to our eye, of which it is deprived ſo ſoon as the true Diſcus
thereof is hid; and in making the obſervation, you ſhall ſee, how
unexpectedly a little cord will cover that reaſonable big body of
light, which ſeemed impoſſible to be hid, unleſſe it were with a
much broader Screene: to meaſure, in the next place, and
ctly to find out, how many of thoſe thickneſſes of the rope
poſe in the diſtance between the ſaid rope and the eye, I take not
onely one diameter of the rope, but laying many pieces of the
ſame together upon a Table, ſo that they touch, I take with a
pair of Compaſſes the whole ſpace occupied by fifteen, or
ty of them, and with that meaſure I commenſurate the diſtance
before with another ſmaller cord taken from the rope to the
courſe of the viſive rayes. And with this ſufficiently-exact
ration I finde the apparent diameter of a fixed Star of the firſt
magnitude, commonly eſteemed to be 2 min. pri. and alſo 3 min.
prim. by Tycho in his Aſtronomical Letters, cap. 167. to be no
more than 5 ſeconds, which is one of the 24. or 36. parts of what
they have held it: ſee now upon what groſſe errours their
ctrines are founded.
of the Star, taketh away the rayes, which belong not to it, but
to our eye, of which it is deprived ſo ſoon as the true Diſcus
thereof is hid; and in making the obſervation, you ſhall ſee, how
unexpectedly a little cord will cover that reaſonable big body of
light, which ſeemed impoſſible to be hid, unleſſe it were with a
much broader Screene: to meaſure, in the next place, and
ctly to find out, how many of thoſe thickneſſes of the rope
poſe in the diſtance between the ſaid rope and the eye, I take not
onely one diameter of the rope, but laying many pieces of the
ſame together upon a Table, ſo that they touch, I take with a
pair of Compaſſes the whole ſpace occupied by fifteen, or
ty of them, and with that meaſure I commenſurate the diſtance
before with another ſmaller cord taken from the rope to the
courſe of the viſive rayes. And with this ſufficiently-exact
ration I finde the apparent diameter of a fixed Star of the firſt
magnitude, commonly eſteemed to be 2 min. pri. and alſo 3 min.
prim. by Tycho in his Aſtronomical Letters, cap. 167. to be no
more than 5 ſeconds, which is one of the 24. or 36. parts of what
they have held it: ſee now upon what groſſe errours their
ctrines are founded.
The diameter of
a fixed ſtar of the
firſt magnitude not
more than five ſec.
min.
a fixed ſtar of the
firſt magnitude not
more than five ſec.
min.
SAGR. I ſee and comprehend this very well, but before we
paſſe any further, I would propound the doubt that ariſeth in
me in the finding the concourſe [or interſection] of the viſual
rayes beyond the eye, when obſervation is made of objects
prehended between very acute angles; and my ſcruple proceeds
from thinking, that the ſaid concourſe may be ſometimes more
remote, and ſometimes leſſe; and this not ſo much, by meanes
of the greater or leſſer magnitude of the object that is beheld, as
becauſe that in obſerving objects of the ſame bigneſſe, it ſeems
to me that the concourſe of the rayes, for certain other
paſſe any further, I would propound the doubt that ariſeth in
me in the finding the concourſe [or interſection] of the viſual
rayes beyond the eye, when obſervation is made of objects
prehended between very acute angles; and my ſcruple proceeds
from thinking, that the ſaid concourſe may be ſometimes more
remote, and ſometimes leſſe; and this not ſo much, by meanes
of the greater or leſſer magnitude of the object that is beheld, as
becauſe that in obſerving objects of the ſame bigneſſe, it ſeems
to me that the concourſe of the rayes, for certain other
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