Barrow, Isaac, Lectiones opticae & geometricae : in quibus phaenomenon opticorum genuinae rationes investigantur, ac exponuntur: et generalia curvarum linearum symptomata declarantur

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318125 rectâ DB, ſit DB. R: : R. BF (ſit autem BF, ut & DHipſi DB
perpendicularis) tum per F, angulo BDHincluſa, tranſeat _hyperbola_
FXX;
ſitque ſpatium BFXI (poſitâ nempe IX ad BF _parallelâ_)
æquale duplo ſpatio ZDL;
ſit denuò DM = DG; erit Min cur-
va quæſita;
quam utique ſi tangat recta TM, erit TD. DM: : R.
DN.
_Probl_. VI.
Sit rurſus ſpatium EDG (ut in præcedente) reperienda eſt curva
AMB, ad quam ſi projiciatur recta DNM, &
ſit DT huic perpen-
11Fig. 188. dicularis, &
MT curvam AMB tangat, fuerit DT = DN.
Adſumatur quæpiam R, & ſit DZ q = {R3/DN}; item acceptâ DB
(cui perpendiculares DH, BF = {R3/DBq};
& per F intra _aſymptotos_
DB, DH deſcribatur _hyperboliformis_ ſecundi generis (in qua nempe
ordinatæ, ceu BF, vel IX, ſint quartæ proportionales in ratione DB
ad R, vel DG ad R) tum capiatur ſpatium BIXF æquale duplo
ZDL;
& ſit DM = DI; erit M in curva quæſita; quam ſi tan-
gat MT, erit DT = DN.
_Probl_. VII
Sit figura quævis ADB (cujus _axis_ AD, _baſis_ DB) & utcunque
22Fig. 189. ductâ PM ad DB parallelâ datum ſit (ſeu expreſſum quomodocunque)
ſpatium APM, oportet hinc ordinatam PM exhibere, vel expri-
mere.
Acceptâ quâqiam R, ſit R x PZ = APM; hinc emergat linea
AZZK;
huic perpendicularis reperiatur ZO; tum erit PZPO
:
: R. PM.
_Exemp_. AP vocetur x & ſit APM = √ r x3, ergo PZ = √
{x3/r};
unde reperietur PO = {3 x x/2 r}. Eſtque √ {x3/r}. {3 x x/2 r}
:
: r. {3/2} √ r x = PM. unde AMB eſt _Parabola_, cujus _Pa-_
rameter eſt {9/4} r.

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