a history of science-2-第51章

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to　an　oblong　form；　was　by　the　second　reduced　into　an　orbicular　one　with　as　much　regularity　as　when　it　did　not　all　pass　through　them。　So　that；　whatever　was　the　cause　of　that　length；　'twas　not　any　contingent　irregularity。　〃I　then　proceeded　to　examine　more　critically　what　might　be　effected　by　the　difference　of　the　incidence　of　rays　coming　from　divers　parts　of　the　sun；　and　to　that　end　measured　the　several　lines　and　angles　belonging　to　the　image。　Its　distance　from　the　hole　or　prism　was　22　feet；　its　utmost　length　13　1/4　inches；　its　breadth　2　5/8；　the　diameter　of　the　hole　1/4　of　an　inch；　the　angle　which　the　rays；　tending　towards　the　middle　of　the　image；　made　with　those　lines；　in　which　they　would　have　proceeded　without　refraction；　was　44　degrees　56'；　and　the　vertical　angle　of　the　prism；　63　degrees　12'。　Also　the　refractions　on　both　sides　of　the　prismthat　is；　of　the　incident　and　emergent　rayswere；　as　near　as　I　could　make　them；　equal；　and　consequently　about　54　degrees　4'；　and　the　rays　fell　perpendicularly　upon　the　wall。　Now；　subducting　the　diameter　of　the　hole　from　the　length　and　breadth　of　the　image；　there　remains　13　inches　the　length；　and　2　3/8　the　breadth；　comprehended　by　those　rays；　which；　passing　through　the　centre　of　the　said　hole；　which　that　breadth　subtended；　was　about　31'；　answerable　to　the　sun's　diameter；　but　the　angle　which　its　length　subtended　was　more　than　five　such　diameters；　namely　2　degrees　49'。　〃Having　made　these　observations；　I　first　computed　from　them　the　refractive　power　of　the　glass；　and　found　it　measured　by　the　ratio　of　the　sines　20　to　31。　And　then；　by　that　ratio；　I　computed　the　refractions　of　two　rays　flowing　from　opposite　parts　of　the　sun's　discus；　so　as　to　differ　31'　in　their　obliquity　of　incidence；　and　found　that　the　emergent　rays　should　have　comprehended　an　angle　of　31'；　as　they　did；　before　they　were　incident。　〃But　because　this　computation　was　founded　on　the　hypothesis　of　the　proportionality　of　the　sines　of　incidence　and　refraction；　which　though　by　my　own　experience　I　could　not　imagine　to　be　so　erroneous　as　to　make　that　angle　but　31'；　which　in　reality　was　2　degrees　49'；　yet　my　curiosity　caused　me　again　to　make　my　prism。　And　having　placed　it　at　my　window；　as　before；　I　observed　that　by　turning　it　a　little　about　its　axis　to　and　fro；　so　as　to　vary　its　obliquity　to　the　light　more　than　an　angle　of　4　degrees　or　5　degrees；　the　colors　were　not　thereby　sensibly　translated　from　their　place　on　the　wall；　and　consequently　by　that　variation　of　incidence　the　quantity　of　refraction　was　not　sensibly　varied。　By　this　experiment；　therefore；　as　well　as　by　the　former　computation；　it　was　evident　that　the　difference　of　the　incidence　of　rays　flowing　from　divers　parts　of　the　sun　could　not　make　them　after　decussation　diverge　at　a　sensibly　greater　angle　than　that　at　which　they　before　converged；　which　being；　at　most；　but　about　31'　or　32'；　there　still　remained　some　other　cause　to　be　found　out；　from　whence　it　could　be　2　degrees　49'。〃　All　this　caused　Newton　to　suspect　that　the　rays；　after　their　trajection　through　the　prism；　moved　in　curved　rather　than　in　straight　lines；　thus　tending　to　be　cast　upon　the　wall　at　different　places　according　to　the　amount　of　this　curve。　His　suspicions　were　increased；　also；　by　happening　to　recall　that　a　tennis…ball　sometimes　describes　such　a　curve　when　〃cut〃　by　a　tennis…racket　striking　the　ball　obliquely。　〃For　a　circular　as　well　as　a　progressive　motion　being　communicated　to　it　by　the　stroke；〃　he　says；　〃its　parts　on　that　side　where　the　motions　conspire　must　press　and　beat　the　contiguous　air　more　violently　than　on　the　other；　and　there　excite　a　reluctancy　and　reaction　of　the　air　proportionately　greater。　And　for　the　same　reason；　if　the　rays　of　light　should　possibly　be　globular　bodies；　and　by　their　oblique　passage　out　of　one　medium　into　another　acquire　a　circulating　motion；　they　ought　to　feel　the　greater　resistance　from　the　ambient　ether　on　that　side　where　the　motions　conspire；　and　thence　be　continually　bowed　to　the　other。　But　notwithstanding　this　plausible　ground　of　suspicion；　when　I　came　to　examine　it　I　could　observe　no　such　curvity　in　them。　And；　besides　（which　was　enough　for　my　purpose）；　I　observed　that　the　difference　'twixt　the　length　of　the　image　and　diameter　of　the　hole　through　which　the　light　was　transmitted　was　proportionable　to　their　distance。　〃The　gradual　removal　of　these　suspicions　at　length　led　me　to　the　experimentum　crucis；　which　was　this：　I　took　two　boards；　and；　placing　one　of　them　close　behind　the　prism　at　the　window；　so　that　the　light　must　pass　through　a　small　hole；　made　in　it　for　the　purpose；　and　fall　on　the　other　board；　which　I　placed　at　about　twelve　feet　distance；　having　first　made　a　small　hole　in　it　also；　for　some　of　the　incident　light　to　pass　through。　Then　I　placed　another　prism　behind　this　second　board；　so　that　the　light　trajected　through　both　the　boards　might　pass　through　that　also；　and　be　again　refracted　before　it　arrived　at　the　wall。　This　done；　I　took　the　first　prism　in　my　hands　and　turned　it　to　and　fro　slowly　about　its　axis；　so　much　as　to　make　the　several　parts　of　the　image；　cast　on　the　second　board；　successively　pass　through　the　hole　in　it；　that　I　might　observe　to　what　places　on　the　wall　the　second　prism　would　refract　them。　And　I　saw　by　the　variation　of　these　places　that　the　light；　tending　to　that　end　of　the　image　towards　which　the　refraction　of　the　first　prism　was　made；　did　in　the　second　prism　suffer　a　refraction　considerably　greater　than　the　light　tending　to　the　other　end。　And　so　the　true　cause　of　the　length　of　that　image　was　detected　to　be　no　other　than　that　LIGHT　consists　of　RAYS　DIFFERENTLY　REFRANGIBLE；　which；　without　any　respect　to　a　difference　in　their　incidence；　were；　according　to　their　degrees　of　refrangibility；　transmitted　towards　divers　parts　of　the　wall。〃＇1＇

THE　NATURE　OF　COLOR　Having　thus　proved　the　composition　of　light；　Newton　took　up　an　exhaustive　discussion　as　to　colors；　which　cannot　be　entered　into　at　length　here。　Some　of　his　remarks　on　the　subject　of　compound　colors；　however；　may　be　stated　in　part。　Newton's　views　are　of　particular　interest　in　this　connection；　since；　as　we　have　already　pointed　out；　the　question　as　to　what　constituted　color　could　not　be　agreed　upon　by　the　philosophers。　Some　held　that　color　was　an　integral　part　of　the　substance；　others　maintained　that　it　was　simply　a　reflection　from　the　surface；　and　no　scientific　explanation　had　been　generally　accepted。　Newton　concludes　his　paper　as　follows：　〃I　might　add　more　instances　of　this　nature；　but　I　shall　conclude　with　the　general　one　that　the　colors　of　all　natural　bodies　have　no　other　origin　than　this；　that　they　are　variously　qualified　to　reflect　one　sort　of　light　in　greater　plenty　than　another。　And　this　I　have　experimented　in　a　dark　room　by　illuminating　those　bodies　with　uncompounded　light　of　divers　colors。　For　by　that　means　any　body　may　be　made　to　appear　of　any　color。　They　have　there　no　appropriate　color；　but　ever　appear　of　the　color　of　the　light　cast　upon　them；　but　yet　with　this　difference；　that　they　are　most　brisk　and　vivid　in　the　light　of　their　own　daylight　color。　Minium　appeareth　there　of　any　color　indifferently　with　which　'tis　illustrated；　but　yet　most　luminous　in　red；　and　so　Bise　appeareth　indifferently　of　any　col