The　doctor　tapped　the　advertisement　with　his　forefinger　impressively。
  〃Fifty　pounds！〃　he　repeated。　〃There　isn't　a　person　in　the　world　to　whom　the　knowledge　of　my　movements　is　worth　fifty　poundsexcept〃
  〃Except？〃　the　Prince　murmured。
  〃Except　Mr。　Inspector　Jacks；〃　Dr。　Whiles　said　slowly。
  The　Prince　seemed　scarcely　to　grasp　the　situation。
  〃Well；〃　he　said；　〃fifty　pounds　is　not　a　great　deal　of　money。　Some　unknown　personpossibly；　as　you　suggest；　Mr。　Jacksis　willing　to　give　fifty　pounds　to　discover　your　whereabouts。　I；　on　the　other　hand；　am　giving　a　thousand　guineas　to　keep　you　here　as　my　guest。　The　odds　do　not　seem　even；　do　they？〃
  〃Put　in　that　way；〃　Dr。　Whiles　admitted；　〃they　certainly　do　not。　But　there　is　another　thing　which　has　come　into　my　mind。〃
  The　Prince　smiled　and　helped　himself　to　one　of　the　very　excellent　cigarettes　which　had　been　provided　for　the　delectation　of　his　visitor。
  〃Pray　treat　me　with　every　confidence；　Dr。　Whiles；〃　he　said。　〃Tell　me　exactly　what　is　in　your　thoughts。〃
  〃Well；　then；　I　will；〃　the　doctor　answered。　〃Sitting　here　with　nothing　particular　to　do；　one　has　plenty　of　leisure　to　think。　For　the　first　time；　I　have　seriously　tried　to　puzzle　out　what　Mr。　Inspector　Jacks　really　wanted　with　me；　why　he　came　down　to　ask　me　about　the　person　whom　I　treated　for　injuries　resulting　from　a　bicycle　accident　one　Wednesday　evening　not　long　ago；　why　he　took　me　up　to　London　to　see　if　I　could　identify　that　person　in　a　very　different　guise。　I　have　tried　to　put　the　pieces　together　and　to　ask　myself　what　he　meant　by　it　all。〃
  〃With　so　much　time　upon　your　hands；　Dr。　Whiles；〃　the　Prince　remarked；　〃you　can　scarcely　fail　to　have　arrived　at　some　reasonable　explanation。〃
  〃I　don't　know　whether　it　is　reasonable　or　not；〃　the　doctor　answered；　〃but　the　obvious　explanation　is　getting　on　my　nerves。　There　are　two　things　which　I　cannot　get　away　from。　One　is　that　I　cannot　for　the　life　of　me　imagine　your　riding　a　bicycle　twelve　or　fifteen　miles　north　of　London　between　eleven　o'clock　and　midnight；　and　the　other〃
  〃Come；　the　other？〃　the　Prince　remarked　encouragingly。
  〃The　other；〃　the　doctor　continued；　〃is　the　fact　that　within　half　a　mile　of　my　house　runs　the　main　London　and　North　Western　line。〃
  〃The　London　and　North　Western　Railway　line；〃　the　Prince　repeated；　〃and　what　has　that　to　do　with　it？〃
  〃This　much；〃　the　doctor　answered；　〃that　on　that　very　night；　about　half　an　hour　before　yourshall　we　call　it　bicycle　accident？the　special　train　from　Liverpool　to　London　passed　along　that　line。　You　will　remember　the　tragic　occurrence　which　took　place　before　she　reached　London；　the　murder　of　the　man　Hamilton　Fynes。　If　you　read　the　report　of　the　evidence　at　the　inquest；　you　will　notice　the　engine　driver's　declaration　that　the　only　time　on　the　whole　journey　when　he　travelled　at　less　than　forty　miles　an　hour　was　when　passing　over　the　viaduct　and　before　entering　the　tunnel　which　is　plainly　visible　from　my　house。〃
  〃This　is　very　interesting；〃　the　Prince　remarked；　〃but　it　is　not　new。　We　have　known　all　this　before。　Perhaps；　though；　some　fresh　thing　has　come　into　your　mind　connected　with　these　happenings。　If　so；　please　do　not　hesitate。　Let　me　hear　it。〃
  〃It　is　a　fresh　thing　to　me；〃　the　doctor　said；〃fresh；　in　a　sense；　though　all　the　time　I　have　had　an　uneasy　feeling　at　the　back　of　my　head。　I　know　now　what　it　was　which　brought　Inspector　Jacks　to　see　me。　I　know　now　what　it　was　he　had　at　the　back　of　his　head　concerning　the　man　who　met　with　a　bicycle　accident　at　this　psychological　moment。〃
  〃Inspector　Jacks　is　a　very　shrewd　fellow；〃　the　Prince　said。　〃I　should　not　be　in　the　least　surprised　if　you　were　entirely　right。〃
  The　doctor　moved　restlessly　in　his　chair。　His　eyes　remained　on　his　companion's　face；　as　though　fascinated。
  〃Can't　you　understand；〃　he　said；　〃that　Inspector　Jacks　is　on　your　track？　Rightly　or　wrongly；　he　believes　that　you　had　something　to　do　with　the　murder　on　the　train　that　night。〃
  The　Prince　nodded　amiably。　He　seemed　in　no　way　discomposed。
  〃I　feel　convinced；〃　he　said；　〃that　you　are　right。　I　agree　with　you。　I　believe　that　Inspector　Jacks　has　had　that　idea　for　some　little　time　now。〃
  The　doctor　gripped　the　sides　of　his　chair　and　stared　at　this　man　who　discussed　a　matter　so　terrible　with　calm　and　perfect　ease。
  〃Yes；　I　have　felt　that　more　than　once；〃　the　Prince　continued。　〃My　presence　upon　the　spot　at　that　precise　moment　with　injuries　which　had　to　be　explained　somehow　or　other；　was；　without　doubt；　unfortunate。〃
  The　two　men　sat　for　several　moments　without　further　speech。　The　doctor's　features　seemed　to　reflect　something　of　the　horror　which　he　undoubtedly　felt。　The　Prince　appeared　only　a　trifle　bored。
  〃So　that　is　why；〃　the　former　exclaimed　hoarsely；　〃I　have　been　appointed　your　physician　in　chief！〃
  〃I　had　given　you　the　credit；　my　dear　doctor；〃　the　Prince　said　smoothly；　〃of　having　arrived　at　that　decision　some　time　ago。　To　a　man　of　your　perceptions　there　can　scarcely　have　been　any　question　about　it　at　all。　Besides；　even　Princes；　you　know；　do　not　give　fees　of　a　thousand　guineas　for　nothing。〃
  Dr。　Whiles　rose　slowly　to　his　feet。
  〃You　know　the　secret　of　that　murder！〃　he　declared。
  〃Why　ask　me？〃　the　Prince　answered。　〃If　I　tell　you　that　I　do；　you　may　find　conscientious　scruples　about　remaining　here。　A　man　is　not　bound；　you　know；　to　give　himself　away。　Make　the　best　of　things；　and　do　not　try　to　see　too　far。〃
  The　doctor　was　looking　a　little　shaken。
  〃If　you　were　mixed　up　in　that　affair；〃　he　said；　〃and　if　I　remain　here　when　my　evidence　is　needed；　I　become　an　accomplice。〃
  〃Only　if　you　remain　here　voluntarily；〃　the　Prince　reminded　him　cheerfully。　〃Remember　that　and　be　comforted。　No　effort　that　you　could　make　now　would　bring　you　into　touch　with　Mr。　Inspector　Jacks　until　I　am　quite　prepared。　So　you　see；　my　dear　doctor；　that　you　have　nothing　with　which　to　reproach　yourself。　I　will　not　insult　you；〃　he　continued；　〃by　suggesting　that　a　reward　of　fifty　pounds　could　possibly　have　influenced　your　attitude。　If　you　have　suffered　your　mind　to　dwell　upon　it　for　a　single　moment；　try　and　remember　the　relative　unimportance　of　such　an　amount　when　compared　with　a　thousand　guineas。〃
  The　doctor　moved　to　the　window　and　back　again。
  〃Supposing；〃　he　said；　〃I　decline　to　remain　here？　Supposing　I　say　that；　believing　you　now　to　have　a　guilty　knowledge　of　this　murder；　I　repudiate　our　bargain？　Supposing　I　say　that　I　will　have　nothing　more　to　do　with　your　thousand　guineas；that　I　will　leave　this　house？〃
  〃Then　we　come　to　close　quarters；〃　the　Prince　answered；　〃and　you　force　me　to　tell　you　in　plain　words　that；　until　I　am　ready　for　you　to　leave　it；　you　are　as　much　a　prisoner　in　this　room　as　though　the　keys　of　the　strongest　fortress　in　Europe　were　turned　upon　you。　I　have　told　you　this　before。　I　thought　that　we　perfectly　understood　one　another。〃
  〃I　did　not　understand；〃　the　doctor　protested。　〃I　knew　that　there　was　trouble；　but　I　did　not　know　that　it　was　this！〃
  〃The　fact　of　your　knowing　or　not　knowing　makes　no　difference；〃　the　Prince　answered。　〃You　are　no　longer　a　free　agent。　The　only　question　for　you　to　decide　is　whether　you　remain　here　willingly　or　whether　you　will　force　me　to　remind　you　of　our　bargain。〃
  The　doctor　was　sitting　down　again　now。　All　the　time　he　watched　the　Prince　with　a　gleam　in　his　eyes；　partly　of　horror；　partly　of　fear。　He　no　longer　doubted　but　that　he　was　in　the　presence　of　a　criminal。
  〃I　am　sorry；〃　the　Prince　continued；　〃that　you　have　allowed　this　little　matter　to　disturb　you。　I　thought　that　we　had　arranged　it　all　at　our　last　interview。　If　you　did　not　surmise　my　reasons　for　keeping　you　here；　then　I　am　afraid　I　gave　you　credit　for　more　intelligence　than　you　possess。　You　will　excuse　me　now；　I　am　sure；〃　he　added；　rising。　〃I　have　some　letters　to　send　off　before　I　change。　By　the　bye；　do　you　care　to　give　me　your　parole？　It　might；　perhaps；　lessen　the　inconvenience　to　which　you　are　unfortunately　subject。〃
  The　doctor　shook　his　head。
  〃No；〃　he　said；　〃I　will　not　give　my　parole！〃
  Late　that　night；　he　tried　the　handle　of　his　door　and　found　it　open。　The　corridor　outside　was　in　thick　darkness。　He　felt　his　way　along　by　the　wall。　Suddenly；　from　behind；　a　pair　of　large　soft　hands　gripped　him　by　the　throat。　Slowly　he　was　drawn　backalmost　strangled。
  〃Let　me　go！〃　he　called　out；　struggling　in　vain　to　find　a　body　upon　which　he　could　gain　a　grip。
  The　grasp　only　tightened。
  〃Back　to　your　rooms！〃　came　a　whisper　through　the　darkness。
  The　doctor　returned。　When　he　staggered　into　his　sitting　room；　he　turned　up　the　electric　light。　There　were　red　marks　upon　his　throat　and　perspiration　upon　his　forehead。　He　opened　the　door　once　more　and　looked　out　upon　the　landing；　striking　a　match　and　holding　it　over　his　head。　There　was　no　one　in　sight；　yet　all　the　time　he　had　the　uncomfortable　feeling　that　he　was　being　watched。　For　the　first　time　in　his　life　he　wondered　whether　a　thousand　guineas　was；　after　all；　such　a　magnificent　fee！
  Almost　at　the　same　time　the　Prince　sat　back　in　the　shadows　of　the　Duchess　of　Devenham's　box　at　the　Opera　and　talked　quietly　to　Lady　Grace。
  〃But　tell　me；　Prince；〃　she　begged；　〃I　know　that　you　are　glad　to　go　home；　but　won't　you　really　miss　this　a　little；the　music；　the　life；　all　these　things　that　make　u