men of invention and industry-第87章

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surpassed　by　the　lens　which　Mr。　Clark　has　in　hand　for　the　Lick

Observatory　（California）；　which　is　to　have　a　clear　aperture　of

three　feet　in　diameter。



＇11＇　Since　the　above　passage　was　written　and　in　type；　I　have　seen

（in　September　1884）　the　reflecting　telescope　referred　to　at　pp。　

357…8。　　It　was　mounted　on　its　cast…iron　equatorial　stand；　and　at

work　in　the　field　adjoining　the　village　green　at　Bainbridge；

Yorkshire。　　The　mirror　of　the　telescope　is　8　inches　in　diameter；

its　focal　length；　5　feet；　and　the　tube　in　which　it　is　mounted；

about　6　feet　long。　　The　instrument　seemed　to　me　to　have　an

excellent　defining　power。



But　Mr。　Lancaster；　like　every　eager　astronomer；　is　anxious　for

further　improvements。　　He　considers　the　achromatic　telescope　the

king　of　instruments；　and　is　now　engaged　in　testing　convex　optical

surfaces；　with　a　view　to　achieving　a　telescope　of　that

description。　　The　chief　difficulty　is　the　heavy　charge　for　the

circular　blocks　of　flint　glass　requisite　for　the　work　which　he

meditates。　　〃That；〃　he　says；　〃is　the　great　difficulty　with

amateurs　of　my　class。〃　　He　has；　however；　already　contrived　and

constructed　a　machine　for　grinding　and　polishing　the　lenses　in　an

accurate　convex　form；　and　it　works　quite　satisfactorily。　　Mr。

Lancaster　makes　his　own　tools。　　From　the　raw　material；　whether　of

glass　or　steel；　he　produces　the　work　required。　　As　to　tools；　all

that　he　requires　is　a　bar　of　steel　and　fire；　his　fertile　brain

and　busy　hands　do　the　rest。　　I　looked　into　the　little　workshop

behind　his　sitting…room；　and　found　it　full　of　ingenious

adaptations。　　The　turning　lathe　occupies　a　considerable　part　of

it；　but　when　he　requires　more　space；　the　village　smith　with　his

stithy；　and　the　miller　with　his　water…power；　are　always　ready　to

help　him。　　His　tools；　though　not　showy；　are　effective。　　His　best

lenses　are　made　by　himself：　　those　which　he　buys　are　not　to　be

depended　upon。　　The　best　flint　glass　is　obtained　from　Paris　in

blocks；　which　he　divides；　grinds；　and　polishes　to　perfect　form。



I　was　attracted　by　a　newly　made　machine；　placed　on　a　table　in　the

sitting…room；　and　on　inquiry　found　that　its　object　was　to　grind

and　polish　lenses。　　Mr。　Lancaster　explained　that　the　difficulty

to　be　overcome　in　a　good　machine；　is　to　make　the　emery　cut　the

surface　equally　from　centre　to　edge　of　the　lens；　so　that　the　lens

will　neither　lengthen　nor　shorten　the　curve　during　its

production。　　To　quote　his　words：　　〃This　　really　involves　the

problem　of　the　'three　bodies；'　or　disturbing　forces　so　celebrated

in　dynamical　mathematics；　and　it　is　further　complicated　by

another　quantity；　the　'coefficient　of　attrition；'　or　work　done　by

the　grinding　material；　as　well　as　the　mischief　done　by　capillary

attractionand　nodal　points　of　superimposed　curves　in　the　path　of

the　tool。　　These　complications　tend　to　cause　rings　or　waves　of

unequal　wear　in　the　surface　of　the　glass；　and　ruin　the　defining

power　of　the　lens；　which　depends　upon　the　uniformity　of　its

curve。　　As　the　outcome　of　much　practical　experiment；　combined

with　mathematical　research；　I　settled　upon　the　ratio　of　speed

between　the　sheave　of　the　lens…tool　guide　and　the　turn…table；

between　whose　limits　the　practical　equalization　of　wear　（or　cut

of　the　emery）　might　with　the　greater　facility　be　adjusted；　by

means　of　varying　the　stroke　and　eccentricity　of　the　tool。　　As　the

result　of　these　considerations　in　the　construction　of　the

machine；　the　surface　of　the　glass　'comes　up'　regularly　all　over

the　lens；　and　the　polishing　only　takes　a　few　minutes'　workthus

keeping　the　truth　of　surface　gained　by　using　a　rigid　tool。〃



The　machine　in　question　consists　of　a　revolving　sheave　or　ring；

with　a　sliding　strip　across　its　diameter；　the　said　strip　having　a

slot　and　clamping　screw　at　one　end；　and　a　hole　towards　the　other；

through　which　passes　the　axis　of　the　tool　used　in　forming　the

lens；the　slot　in　the　strip　allowing　the　tool　to　give　any　stroke

from　0　to　1。25　inch。　　The　lens　is　carried　on　a　revolving

turn…table；　with　an　arrangement　to　allow　the　axis　of　the　lens　to

coincide　with　the　axis　of　the　table。　　The　ratio　of　speed　between

the　sheave　and　turn…table　is　arranged　by　belt　and　properly　sized

pulleys；　and　the　whole　can　be　driven　either　by　hand　or　by　power。　

The　sheave　merely　serves　as　a　guide　to　the　tool　in　its　path；　and

the　lens　may　either　be　worked　on　the　turn…table　or　upon　a　chuck

attached　to　the　tool　rod。　　The　work　upon　the　lens　is　thus　to　a

great　extent　independent　of　the　error　of　the　machine　through

shaking；　or　bad　fitting；　or　wear；　and　the　only　part　of　the

machine　which　requires　really　first…class　work　is　the　axis　of　the

turn…table；　which　（in　this　machine）　is　a　conical　bearing　at　top；

with　steel　centre　below；the　bearing　turned；　hardened；　and　then

ground　up　true；　and　run　in　anti…friction　metal。　　Other　details

might　be　given；　but　these　are　probably　enough　for　present

purposes。　　We　hope；　at　some　future　time；　for　a　special　detail　of

Mr。　Lancaster's　interesting　investigations；　from　his　own　mind　and

pen。



＇12＇　The　translations　are　made　by　W。　Cadwalladr　Davies；　Esq。



＇13＇　This　evidence　was　given　by　Mr。　W。　Cadwalladr　Davies　on　the

28th　October；　1880。











End