 crystals" (1906, 35). Hence, on Duhem's view, the credit for Huygens's account of double refraction goes only to the representative part, whereas the wave hypothesis did not contribute at all to this predictive success. Things, however, are more complicated. Duhem himself showed this when he described how Huygens used the Iceland Spar to study double refraction. Huygens observed, as Bartholin did before him, that two images of one and the same line could be produced by placing a Spar on a piece of paper. Moreover, one of these images would rotate if the Spar itself was being rotated. One of these images, that Huygens called "ordinary", obeys Descartes's law and stays fixed when the Spar is rotated. The rotating image, called "extraordinary", does not satisfy the law of refraction. Upon having explained the phenomena of reflection Marie Gueguen and Stathis Psillos Anti-Scepticism and Epistemic Humility in Pierre Duhem's Philosophy of Science 62 and refraction based on a wave hypothesis and the propagation of light through spherical waves, Huygens postulated that the two images were the results of two different kinds of wave propagation, corresponding respectively to the light propagation in the aether contained in the Spar and to the light propagation in the particles constituting the Spar itself. While the former waves are spherical, the latter are ellipsoidal, thus explaining the phenomena of double refraction. Based on these explanatory hypotheses which yielded the ellipsoidal model, Huygens thus constructed a geometrical model that succeeded in representing the phenomena of double refraction and thus extended Descartes's theory of refraction. The point here that Huygens did use the wave hypothesis to construct his theory of double refraction. It was the wave hypothesis that made possible the ellipsoidal model, since as he (1690b, 73) put it: "upon having explained the refraction of transparent ordinary bodies, by means of the spherical emanations of light, (...), I went back to examining the nature of this crystal, about which I could not discover anything before" (M. G. translation). An ellipsoidal propagation as the one described would not have made sense outside the wave theory. Hence, that we may, ex post facto, deem a hypothesis 'explanatory' does not imply that, in the reasoning that led to this model, it did not play a role. Duhem, indeed, grants that H
crystals " (1906, 35). Hence, on Duhem's view, the credit for Huygens's explanation of bivalent refraction goes merely to the representative function, whereas the wave hypothesis did not contribute at wholly to this predictive success. Things, however, are more complicated. Duhem himself showed this when he describe how Huygens used the Iceland Spar to study bivalent refraction. Huygens observed, as Bartholin did before him, that two images of one and the like line could be produced by rate a Spar on a piece of paper. furthermore, one of these images would rotate if the Spar itself was being rotated. One of these persona, that Huygens called " ordinary ", obey Descartes's law and stay fixed when the Spar is rotate. The rotating image, called " extraordinary ", does not meet the law of refraction. Upon having explained the phenomena of reflection Marie Gueguen and Stathis Psillos Anti - Scepticism and Epistemic Humility in Pierre Duhem's Philosophy of Science 62 and refraction free-base on a wave hypothesis and the propagation of light through spherical waves, Huygens postulated that the two images were the results of two different kinds of wave generation, match respectively to the light generation in the ether contained in the Spar and to the light propagation in the particles form the Spar itself. While the former waves are spherical, the latter are ellipsoidal, therefore explaining the phenomena of double refraction. Based on these explanatory hypotheses which yielded the ellipsoidal model, Huygens thus constructed a geometrical exemplar that succeeded in represent the phenomena of double refraction and thus extended Descartes's hypothesis of refraction. The point here that Huygens did use the wave hypothesis to construct his theory of double refraction. It was the wave hypothesis that take possible the ellipsoid model, since as he (1690b, 73) put it: " upon having explained the refraction of transparent average bodies, by means of the spherical emanations of lighter, (...), I went back to examining the nature of this crystal, about which I could not discover anything before " (M. G. translation). An ellipsoid propagation as the one described would not have made common sense outside the wave theory. Hence, that we may, ex post facto, deem a hypothesis' explanatory' does not imply that, in the reasoning that lead to this exemplar, it did not play a role. Duhem, indeed, allow that H
 crjstals" (1906, 35). Hence, on Duhem's view, the credij dor Hubgens's zccount uf double refraction goes onpy to tye representative part, whereas nhe wave yypouhesis did not coifribute at alm to chms predictive soccess. Thingv, however, are kofe complicated. Duhem himself showed tris whem je described hjw Hlydens lstd the Iceland Spar to study doubme refrection. Huygens pbserved, as Bartholin did hefoge him, that two imwges of one and rhe same lind could be produced by placing a Spar on a piece of pxper. Koreover, obe of jhese images woulq rotate if bne Spas itselg was being robated. Onw of these images, thav Huygens called "ordynary", obefs Descartes's law abd stayv fifed dyen tht S'ar is rohatxd. The rotafing image, xalled "extraordinaru", qies not satisry the lww of refraction. Upon having explained uhe pgenomena of reflection Narie Gueguen and Stajhis Psilljs Anti-Scepticism and Epistemic Humility in Pierra Duhxm'r Pkljuwoohy of Science 62 and refraction based on a wavq huppthesis and thc propagation of logjt jhrough spherizal warss, Huygens postulatef that jhe twi images rere the results of two differebt kinds of cavw propagation, corrzsponding rerpecjively to the light propagatiun ih the aetheg containsa in the Spar ana tp dhe lighu propagation in tre particoes eonstituging the S[ar itself. Whilc the former waves wre skhericdl, the lather are ellipsoidal, thus explaiimng the phenoketa mf doublz refrsction. Based jn these explauatory hvpotheres which jielded tie ellipsoidwl model, Huygajs thus consvructed a geonetrucal moadl that succeeced in reirtsenting tye phenomena of domble terraction and thbr extended Descatter's ehvorb of wafraction. Tha poknt nere ghat Huygenw did use the wave hypothasis to construct his yhcory of diuble resraction. It wss the wave hypothtsis tiat mave posxibje the ellipsoidal model, since as he (1690b, 73) pub it: "upon havyng cxplwined the xefraction of transparent ordinary bodiew, by means of the wpherical emanationx of light, (...), M went back to axamining the nature of this crystal, sbout which I could nkt diswover anything before" (M. G. translation). An ellipsoidal propagation as the one dewcribev rould not hzve kade veuse outsiqe tie wave theory. Henbe, that we may, ex post facto, deeo a hypodhzsis 'explanatory' does not impky that, in the teasoning that led to this model, iy did not play a role. Duhem, indred, grants that H
crystals" (1906, 35). Hence, on Duhem's view, for account of refraction goes only the hypothesis did not at all to predictive success. Things, however, are more Duhem himself showed this when he described how Huygens used the Iceland Spar study double refraction. Huygens observed, as Bartholin did before him, that two images one the line be produced by placing a Spar on a piece of paper. Moreover, one of these images rotate if the Spar itself was being rotated. of these images, that called "ordinary", obeys Descartes's law stays when the is The image, called "extraordinary", not satisfy the law of refraction. Upon having explained the phenomena of reflection Marie Gueguen and Stathis Anti-Scepticism and in Pierre Philosophy Science and refraction based wave hypothesis and the propagation of waves, Huygens postulated that the two images were results of different kinds of wave propagation, corresponding to the light propagation in the aether contained the Spar and to the light propagation in the particles constituting the Spar itself. While waves are spherical, the are ellipsoidal, thus the of refraction. on these hypotheses which yielded the ellipsoidal model, Huygens thus constructed a geometrical that succeeded in representing the phenomena of double refraction and Descartes's of refraction. The here that Huygens did the hypothesis to construct his double It hypothesis made the ellipsoidal model, since he (1690b, 73) put it: having explained the refraction means of the spherical emanations of light, (...), went back to examining the nature of crystal, about which I could not discover anything before" (M. G. translation). ellipsoidal propagation one described would not have made sense outside wave theory. Hence, that may, ex post facto, deem a hypothesis 'explanatory' does imply in the that led to model, it did play a role. grants that
 crystals" (1906, 35). Hence, on Duhem's view, The credit fOr HuyGenS's aCcOunt Of doUble refraction GOes oNly to the representative Part, wHeREas tHE wAve hyPothesiS DiD NOt cOnTrIbuTe AT aLl to tHis PredictIve success. thiNgS, however, are mORe ComplicateD. DuHem himself shOweD this wHeN he DEscriBed How HuYgens uSEd the ICeland SpaR tO Study dOUble refRACtIon. HUygens observed, as BARtHOlin did before hIm, that TwO ImAGEs oF onE and the samE lIne coULd be proDUcED BY plACing a Spar on a pIece of paper. mOreOver, onE oF thESe imagEs wouLd ROtaTe if the Spar ItseLf was beinG rotatED. One of tHEse imagEs, that huyGenS calLEd "OrDinArY", ObeYS DEscARteS's law and StAyS fixeD wheN THE spar Is rOtatEd. The Rotating image, CalLed "eXTraOrdinAry", doEs noT sAtisfY the laW of reFrAction. Upon havinG expLained the PheNoMenA oF reflECtion MAriE GuEguen anD StathiS psiLlOS aNtI-Scepticism and EpisTeMIC HUmility iN PierrE duHeM'S PhilosoPhY of scieNCE 62 and rEfraCTiOn based oN a wave HYpOtHesis anD tHe propAgAtiOn oF lighT ThroUgh sphErical waVes, HuYGens postulated THat the two imagES wERE tHE resUltS of two diffeRent KInds Of waVE pRopAGatioN, corrEsPOnDIng respectively to thE lIght prOpagaTion in the aethEr containeD IN The Spar aNd to THe LIght propagatioN in thE particles COnstitutIng thE Spar itsElf. While tHE Former waVes Are SphEriCAL, tHe latter are elLIPsoiDaL, thus exPlaIning thE phEnoMenA of DoUble refraCtion. BasEd On ThEsE exPlanaTOry hypotHeSes WhIch YieldED the elLipsoIdal MoDeL, huyGens thuS CoNSTrucTeD a GeomEtrIcAl modEl thAT suCceeded In represeNtiNG the PhEnOmena of Double refractIoN and thus exTeNdeD DescaRTEs's theorY of refraction. The point heRE that HuYgeNs did Use tHe wave hypOthEsis to ConSTruct hIs theoRy of dOuBle REFractION. IT waS tHe wave hypoTHEsiS that MaDe poSsible tHe ellipsoidal model, SIncE as he (1690b, 73) put it: "upOn hAvinG EXpLaiNEd THe rEfRActION of transparent oRdinary bodIeS, By Means of the SPheRiCal emanAtions oF lighT, (...), i went baCk to examiNing the naTuRe of THIs cRystal, abouT which I cOuld not diSCover ANyThing BefOre" (M. G. tRaNslAtion). an elliPSoiDal prOpagatIoN as the One deScRibed wouLd not have made sense outsiDe the wAve thEorY. Hence, thaT we MAy, eX post factO, deeM a hypothesIs 'eXplAnatoRy' dOEs not ImplY ThAt, iN The reAsonINg that led TO tHis MODeL, it did not plAY A RolE. DuheM, inDEed, graNts tHat H
 crystals" (1906, 35). Hen ce, on Duh em'svie w,th e cr edit  for Huygens's acco unt of double refracti on go es only to  thereprese n ta t i vepa rt , w he r ea s the  wa ve hypo thesis did  no tcontribute a t  a ll to this  pr edictive suc ces s. Thi ng s,h oweve r,are m ore co m plicat ed. Duhem  h i mselfs howed t h i swhen  he described how Hu y gens used theIcelan dS pa r tostu dy doublere fract i on. Huy g en s o bse r ved, as Barth olin did be f ore  him,th att wo ima ges o fo neand the sam e li ne couldbe pro d uced by placing  a Spa r o n a  pie c eof  pa pe r . M o re ove r , o ne of th es eimage s wo u l d rota teif t he Sp ar itself was  be ingr ota ted.One o f th es e ima ges, t hat H uy gens called "or dina ry", obey s D es car te s's l a w andsta ysfixed w hen the Spa ri s ro tated. The rotatin gi m ag e, calle d "ext r ao rd i nary", d oe s n ot s a t isfythel aw  of refr action .  U po n havin gexplai ne d t hepheno m enaof ref lectionMarie Gueguen and St a this PsillosA nt i - Sc e ptic ism  and Episte micH umil ityi nPie r re Du hem's  P h il o sophy of Science 62  a nd ref racti on based on a  wave hypo t h e sis andthep ro p agation of lig ht th rough sphe r ical wav es, H uygens p ostulated t hat thetwo  im age s w e r ethe results o f twodi fferent  ki nds ofwav e p rop aga ti on, corre sponding  r es pe ct ive ly to the ligh tpro pa gat ion i n  the a ether  con ta in e d i n the S p ar a nd t oth e li ght  p ropag atio n  in  the pa rticles c ons t itut in gthe Spa r itself. Whi le  the forme rwav es are s pherical , the latter are ellips o idal, t hus  expl aini ng the ph eno mena o f d o uble r efract ion.Ba sed o n the s e  e xpl an atory hypo t h ese s whi ch  yie lded th e ellipsoidal mode l , H uygens thus c ons truc t e da g e om e tri ca l  mo d e l that succeede d in repre se n ti ng the phe n ome na  of dou ble ref racti o n and t hus exten ded Desca rt es's t heo ry of refr action.The point heret ha t Huy gen s didus e t he wa ve hyp o the sis t o cons tr uct hi s the or y of dou ble refraction. It wasthe wa ve hy pot hesis tha t m a depossibletheellipsoida l m ode l, si nce as he  (16 9 0b , 7 3 ) put  it: "upon hav i ng  ex p l ai ned the ref r a c tio n oftra n sparen t or dinary bodies, by means of the s pher i c alema n atio ns  of light, (.. .),  I w ent back  t o examining  the nat ur e  of t his cr ystal,  aboutw h ic h  I cou ld n otdiscoverany th i ng befo re "( M. G.tran sl ation) . An e l lips o i dal propagationas th e one d e scr ibedwo uld not have  made sens e outside t he wav e th eory.  Hence,  t hat we  ma y,  ex post f a cto, deem  a hy pothesi s'exp lan atory'  doe s not i mply  t hat , in ther e as o ni ng tha t le d toth is m odel, itd id not p lay a role.  D uhe m ,  indee d ,g r ants thatH
 crystals"_(1906, 35)._Hence, on Duhem's view,_the credit_for_Huygens's account_of_double refraction goes_only to the_representative part, whereas the_wave hypothesis did_not_contribute at all to this predictive success. Things, however, are more complicated. Duhem himself_showed_this when_he_described_how Huygens used the Iceland_Spar to study double refraction._Huygens observed,_as Bartholin did before him, that two images_of_one and the_same line could be produced by placing a Spar_on a piece of paper. Moreover,_one of these_images_would_rotate if the Spar_itself was being rotated. One of_these images, that Huygens called "ordinary",_obeys Descartes's law and stays fixed when_the Spar is rotated. The rotating_image, called "extraordinary", does not_satisfy the_law of refraction. Upon having_explained the phenomena_of reflection_Marie Gueguen and_Stathis Psillos Anti-Scepticism and Epistemic Humility_in Pierre Duhem's_Philosophy of Science 62 and refraction_based_on a wave_hypothesis_and_the propagation_of light through_spherical_waves, Huygens_postulated_that the two images were the_results_of two different kinds of wave propagation,_corresponding respectively to the_light_propagation in the aether_contained in the Spar and_to the light propagation in the_particles constituting_the Spar_itself. While the former waves are spherical, the latter are ellipsoidal,_thus explaining the phenomena of double_refraction. Based on these_explanatory hypotheses_which_yielded the ellipsoidal_model,_Huygens thus_constructed a geometrical model that succeeded in_representing the_phenomena of double refraction and thus_extended Descartes's theory of_refraction._The point here that Huygens did_use the wave hypothesis to construct_his theory of double refraction._It_was_the wave hypothesis that made_possible the ellipsoidal model, since as_he (1690b, 73)_put it: "upon having explained the refraction_of_transparent ordinary bodies, by means of_the_spherical emanations of light, (...), I_went_back_to examining the nature of_this crystal, about which I could_not discover anything before" (M. G. translation). An ellipsoidal_propagation as the_one described would not have_made_sense_outside the wave theory. Hence, that we may, ex post_facto, deem_a hypothesis 'explanatory'_does not imply that, in the reasoning that led to_this model, it did not play a_role. Duhem, indeed, grants that H