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

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27481 rectam AP ſecare ad T; ut ipſius jam rectæ PT quantitatem exqui-
11Fig. 115. ram;
curvæ arcum MN indefinitè parvum ſtatuo; tum duco rectas
NQ ad MP, &
NR ad AP parallelas; nomino MP = _m_; PT
= _t_;
MR = _a_; NR = _e_; reliquáſque rectas, ex ſpeciali curvæ
natura determinatas, utiles propoſito, nominibus deſigno;
ipſas au-
tem MR, NR (&
mediantibus illis ipſas MP, PT) per _æquationem_
è Calculo deprehenſam inter ſe comparo;
regulas interim has obſer-
vans.
1. Inter computandum omnes abjicio terminos, in quibus
ipſarum _a_, vel _e_ poteſtas habetur, vel in quibus ipſæ ducuntur in ſe
(etenim iſti termini nihil valebunt).
2. Poſt _æquationem constitutam_, omnes abjicio terminos, literis
conftantes quantitates notas, ſeu determinatas deſignantibus;
aut in
quibus non habentur _a_, vel _e_.
(etenim illi termini ſemper, ad unam
æquationis partem adducti, nihilum adæquabunt).
3. Pro _a_ ipſam _m_; (vel MP) pro _e_ ipſam _t_ (vel PT) ſubſtituo.
Hinc demùm ipſius PT quantitas dignoſcetur.
Quòd ſi calculum ingrediatur curvæ cujuſpiam indefinita particula;
ſubſtituatur ejus loco tangentis particula ritè ſumpta; vel ei quævis
(ob indefinitam curvæ parvitatem) æquipollens recta.
Hæc autem è ſubnexis Exemplis clariùs eluceſcent.
Exemp. I.
Angulus ABH rectus ſit; & ſit curva AMO talis, ut per A du-
ctâ utcunque rectâ AK, quæ rectam BH ſecet in K, curvam AMO
22Fig. 116. in M, ſit ſemper ſubtenſa AM æqualis abſciſſæ BK;
hujus curvæ ad
M tangens eſt deſignanda.
Fiant quæ ſuprà præſcripta ſunt, & (ductâ ANL) nominetur
AB = _r_;
& AP = _q_; unde AQ = _q_ - _e_; item QN = _m_ -
_a_.
ergò eſt _qq_ + _ee_ - 2 _qe_ + _mm_ + _aa_ - 2 _ma_ = (AQq
+ QNq = ANq = ) BLq;
hoc eſt (rejectis, uti monitum eſt,
rejiciendis) _qq_ - 2 _qe_ + _mm_ - 2 _ma_ = BLq.
Porrò eſt
AQ.
QN: : AB. BL; hoc eſt _q_ - _e. m_ - _a_: : _r._ BL =
{_rm_ - _ra_.
/_q_ - _e_} quare {_rrmm_ + _rraa_ - 2 _rrma_/_qq_ + _ee_ - 2 _qe_. } = BLq; ſeu
(rejectis ſuperfluis) {_rrmm_ - 2 _rrma_/_qq_ - 2 _qe@_} = BLq = _qq_ - 2 _qe_ +
_mm_ - 2 _ma_.
vel _rrmm_ - 2 _rrma_ = _q_4 - 2 _q_3_e_ + _qqmm_ - 2 _qqma_ - 2 _q_3_e_ +
4 _qqee_ - 2 _qmme_ + 4 _qmae_;
hoc eſt (abjectis iis, quæ

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