a history of science-4-第20节

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


the　two　lenses　with　a　cement　having　the　same　index　of　refraction　as　the　lenses　themselvesan　improvement　of　manifest　advantage。　An　improvement　yet　more　important　was　made　by　Dr。　Wollaston　himself　in　the　introduction　of　the　diaphragm　to　limit　the　field　of　vision　between　the　lenses；　instead　of　in　front　of　the　anterior　lens。　　A　pair　of　lenses　thus　equipped　Dr。　Wollaston　called　the　periscopic　microscope。　Dr。　Brewster　suggested　that　in　such　a　lens　the　same　object　might　be　attained　with　greater　ease　by　grinding　an　equatorial　groove　about　a　thick　or　globular　lens　and　filling　the　groove　with　an　opaque　cement。　This　arrangement　found　much　favor；　and　came　subsequently　to　be　known　as　a　Coddington　lens；　though　Mr。　Coddington　laid　no　claim　to　being　its　inventor。

Sir　John　Herschel；　another　of　the　very　great　physicists　of　the　time；　also　gave　attention　to　the　problem　of　improving　the　microscope；　and　in　1821　he　introduced　what　was　called　an　aplanatic　combination　of　lenses；　in　which；　as　the　name　implies；　the　spherical　aberration　was　largely　done　away　with。　It　was　thought　that　the　use　of　this　Herschel　aplanatic　combination　as　an　eyepiece；　combined　with　the　Wollaston　doublet　for　the　objective；　came　as　near　perfection　as　the　compound　microscope　was　likely　soon　to　come。　But　in　reality　the　instrument　thus　constructed；　though　doubtless　superior　to　any　predecessor；　was　so　defective　that　for　practical　purposes　the　simple　microscope；　such　as　the　doublet　or　the　Coddington；　was　preferable　to　the　more　complicated　one。

Many　opticians；　indeed；　quite　despaired　of　ever　being　able　to　make　a　satisfactory　refracting　compound　microscope；　and　some　of　them　had　taken　up　anew　Sir　Isaac　Newton's　suggestion　in　reference　to　a　reflecting　microscope。　　In　particular；　Professor　Giovanni　Battista　Amici；　a　very　famous　mathematician　and　practical　optician　of　Modena；　succeeded　in　constructing　a　reflecting　microscope　which　was　said　to　be　superior　to　any　compound　microscope　of　the　time；　though　the　events　of　the　ensuing　years　were　destined　to　rob　it　of　all　but　historical　value。　For　there　were　others；　fortunately；　who　did　not　despair　of　the　possibilities　of　the　refracting　microscope；　and　their　efforts　were　destined　before　long　to　be　crowned　with　a　degree　of　success　not　even　dreamed　of　by　any　preceding　generation。

The　man　to　whom　chief　credit　is　due　for　directing　those　final　steps　that　made　the　compound　microscope　a　practical　implement　instead　of　a　scientific　toy　was　the　English　amateur　optician　Joseph　Jackson　Lister。　　Combining　mathematical　knowledge　with　mechanical　ingenuity；　and　having　the　practical　aid　of　the　celebrated　optician　Tulley；　he　devised　formulae　for　the　combination　of　lenses　of　crown　glass　with　others　of　flint　glass；　so　adjusted　that　the　refractive　errors　of　one　were　corrected　or　compensated　by　the　other；　with　the　result　of　producing　lenses　of　hitherto　unequalled　powers　of　definition；　lenses　capable　of　showing　an　image　highly　magnified；　yet　relatively　free　from　those　distortions　and　fringes　of　color　that　had　heretofore　been　so　disastrous　to　true　interpretation　of　magnified　structures。

Lister　had　begun　his　studies　of　the　lens　in　1824；　but　it　was　not　until　1830　that　he　contributed　to　the　Royal　Society　the　famous　paper　detailing　his　theories　and　experiments。　Soon　after　this　various　continental　opticians　who　had　long　been　working　along　similar　lines　took　the　matter　up；　and　their　expositions；　in　particular　that　of　Amici；　introduced　the　improved　compound　microscope　to　the　attention　of　microscopists　everywhere。　And　it　required　but　the　most　casual　trial　to　convince　the　experienced　observers　that　a　new　implement　of　scientific　research　had　been　placed　in　their　hands　which　carried　them　a　long　step　nearer　the　observation　of　the　intimate　physical　processes　which　lie　at　the　foundation　of　vital　phenomena。　　For　the　physiologist　this　perfection　of　the　compound　microscope　had　the　same　significance　that　the；　discovery　of　America　had　for　the　fifteenth…century　geographersit　promised　a　veritable　world　of　utterly　novel　revelations。　Nor　was　the　fulfilment　of　that　promise　long　delayed。

Indeed；　so　numerous　and　so　important　were　the　discoveries　now　made　in　the　realm　of　minute　anatomy　that　the　rise　of　histology　to　the　rank　of　an　independent　science　may　be　said　to　date　from　this　period。　Hitherto；　ever　since　the　discovery　of　magnifying…glasses；　there　had　been　here　and　there　a　man；　such　as　Leuwenhoek　or　Malpighi；　gifted　with　exceptional　vision；　and　perhaps　unusually　happy　in　his　conjectures；　who　made　important　contributions　to　the　knowledge　of　the　minute　structure　of　organic　tissues；　but　now　of　a　sudden　it　became　possible　for　the　veriest　tyro　to　confirm　or　refute　the　laborious　observations　of　these　pioneers；　while　the　skilled　observer　could　step　easily　beyond　the　barriers　of　vision　that　hitherto　were　quite　impassable。　And　so；　naturally　enough；　the　physiologists　of　the　fourth　decade　of　the　nineteenth　century　rushed　as　eagerly　into　the　new　realm　of　the　microscope　as；　for　example；　their　successors　of　to…day　are　exploring　the　realm　of　the　X…ray。

Lister　himself；　who　had　become　an　eager　interrogator　of　the　instrument　he　had　perfected；　made　many　important　discoveries；　the　most　notable　being　his　final　settlement　of　the　long…mooted　question　as　to　the　true　form　of　the　red　corpuscles　of　the　human　blood。　In　reality；　as　everybody　knows　nowadays；　these　are　biconcave　disks；　but　owing　to　their　peculiar　figure　it　is　easily　possible　to　misinterpret　the　appearances　they　present　when　seen　through　a　poor　lens；　and　though　Dr。　Thomas　Young　and　various　other　observers　had　come　very　near　the　truth　regarding　them；　unanimity　of　opinion　was　possible　only　after　the　verdict　of　the　perfected　microscope　was　given。

These　blood　corpuscles　are　so　infinitesimal　in　size　that　something　like　five　millions　of　them　are　found　in　each　cubic　millimetre　of　the　blood；　yet　they　are　isolated　particles；　each　having；　so　to　speak；　its　own　personality。　　This；　of　course；　had　been　known　to　microscopists　since　the　days　of　the　earliest　lenses。　It　had　been　noticed；　too；　by　here　and　there　an　observer；　that　certain　of　the　solid　tissues　seemed　to　present　something　of　a　granular　texture；　as　if　they；　too；　in　their　ultimate　constitution；　were　made　up　of　particles。　　And　now；　as　better　and　better　lenses　were　constructed；　this　idea　gained　ground　constantly；　though　for　a　time　no　one　saw　its　full　significance。　In　the　case　of　vegetable　tissues；　indeed；　the　fact　that　little　particles　encased　a　membranous　covering；　and　called　cells；　are　the　ultimate　visible　units　of　structure　had　long　been　known。　But　it　was　supposed　that　animal　tissues　differed　radically　from　this　construction。　　The　elementary　particles　of　vegetables　〃were　regarded　to　a　certain　extent　as　individuals　which　composed　the　entire　plant；　while；　on　the　other　hand；　no　such　view　was　taken　of　the　elementary　parts　of　animals。〃


ROBERT　BROWN　AND　THE　CELL　NUCLEUS

In　the　year　1833　a　further　insight　into　the　nature　of　the　ultimate　particles　of　plants　was　gained　through　the　observation　of　the　English　microscopist　Robert　Brown；　who；　in　the　course　of　his　microscopic　studies　of　the　epidermis　of　orchids；　discovered　in　the　cells　〃an　opaque　spot；〃　which　he　named　the　nucleus。　　Doubtless　the　same　〃spot〃　had　been　seen　often　enough　before　by　other　observers；　but　Brown　was　the　first　to　recognize　it　as　a　component　part　of　the　vegetable　cell　and　to　give　it　a　name。


〃I　shall　conclude　my　observations　on　Orchideae；〃　said　Brown；　〃with　a　notice　of　some　points　of　their　general　structure；　which　chiefly　relate　to　the　cellular　tissue。　　In　each　cell　of　the　epidermis　of　a　great　part　of　this　family；　especially　of　those　with　membranous　leaves；　a　single　circular　areola；　generally　somewhat　more　opaque　than；　the　membrane　of　the　cell；　is　observable。　This　areola；　which　is　more　or　less　distinctly　granular；　is　slightly　convex；　and　although　it　seems　to　be　on　the　surface　is　in　reality　covered　by　the　outer　lamina　of　the　cell。　There　is　no　regularity　as　to　its　place　in　the　cell；　it　is　not　unfrequently；　however；　central　or　nearly　so。

〃As　only　one　areola　belongs　to　each　cell；　and　as　in　many　cases　where　it　exists　in　the　common　cells　of　the　epidermis；　it　is　also　visible　in　the　cutaneous　glands　or　stomata；　and　in　these　is　always　doubleone　being　on　each　side　of　the　limbit　is　highly　probable　that　the　cutaneous　gland　is　in　all　cases　composed　of　two　cells　of　peculiar　form；　the　line　of　union　being　the　longitudinal　axis　of　the　disk　or　pore。

〃This　areola；　or　nucleus　of　the　cell　as　perhaps　it　might　be　termed；　is　not　confined　to　the　epidermis；　being　also　found；　not　only　in　the　pubescence　of　the　surface；　particularly　when　jointed；　as　in　cypripedium；　but　in　many　cases　in　the　parenchyma　or　internal　cells　of　the　tissue；　especially　when　these　are　free　from　the　deposition　of　granular　matter。

〃In　the　compressed　cells　of　the　epidermis　the　nucleus　is　in　a　corresponding　degree　flattened；　but　in　the　internal　tissue　it　is　often　nearly　spherical；　more　or　less　firmly　adhering　to　one　of　the　walls；　and　projecting　into　the　cavity　of　the　cell。　　In　this　state　it　may　not　unfrequently　be　found。　in　the　substance　of　the　column　and　in　that　of　the　perianthium。

〃The　nucleus　is　manifest　also　in　the　tissue　of　the　stigma；　where　in　accordance　with　the　compression　of　th