Christian Huygens, dated at Paris, to Henry Oldenburg

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                                Paris july 1.1672.
(Enterd L B 5.285.)
Problema Alhazeni 
Dato circulo, cujus centrum A radius AD et punctis duobus BC; invenire 
punctum H in circumferentia circuli dati unde ductae HB, HC faciant ad circumferentiam 
angulos aequales.
Donatur inventum, ductaque AM recta, qua bifariam secet angulum BAC ducat 
ei perpendicularis HF, itemque BM CL jungatur porro AH cui perpend sit HE rectaeque 
BH, HC occurant AM in punctis K, G.
Sit jam AM [proportional to] a
etc.
Quia ergo aequales 
anguli KHE et  CHZ sive 
EHG est que EHA
angulus rectus erit ut 
KE ad EG ita KA ad AG.
Quia vero BM ad MK ut 
HF ad FK erit 
ut BM + HF ad HF ita MF ad FK 
b + y — y — a-x/ay-xy/ b
+y

add FA x
fit KA ay-[text?]/b+y

Rursus quia CL ad LG ut HF ad 
FG erit permutando et dividendo CL - HF ad HF ut LF ad FG 
n - y — y — c-x — cy-xy/n-y, qua 
ablata ab AF xx fit GA nx-cy/ n-y
est autem EA [proportional to] dd/x , quia proportionale 
FA, AH, AE. ergo EA - GA, hoc est EG,
... etc
Sed diximus quod KE ad EG ut KA ad quod
ergo 
... etc

                            
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Manuscript details

Author
Christiaan Huygens
Reference
EL/H1/73
Series
EL
Date
1672
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Christian Huygens, dated at Paris, to Henry Oldenburg, 1672. From The Royal Society, EL/H1/73

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