Achromatic refracting telescopes History of the telescope

light path through achromatic lens.

from time of invention of first refracting telescopes supposed chromatic errors seen in lenses arose errors in spherical figure of surfaces. opticians tried construct lenses of varying forms of curvature correct these errors. isaac newton discovered in 1666 chromatic colors arose un-even refraction of light passed through glass medium. led opticians experiment lenses constructed of more 1 type of glass in attempt canceling errors produced each type of glass. hoped create achromatic lens ; lens focus colors single point, , produce instruments of shorter focal length.

the first person succeeded in making practical achromatic refracting telescope chester moore hall essex, england. argued different humours of human eye refract rays of light produce image on retina free color, , reasonably argued might possible produce result combining lenses composed of different refracting media. after devoting time inquiry found combining 2 lenses formed of different kinds of glass, make achromatic lens effects of unequal refractions of 2 colors of light (red , blue) corrected. in 1733, succeeded in constructing telescope lenses exhibited reduced chromatic aberration. 1 of instruments had objective measuring 2 ⁄2 inches (6.4 cm) relatively short focal length of 20 inches (51 cm).

hall man of independent means , seems have been careless of fame; @ least took no trouble communicate invention world. @ trial in westminster hall patent rights granted john dollond (watkin v. dollond), hall admitted first inventor of achromatic telescope. however, ruled lord mansfield not original inventor ought profit such invention, 1 brought forth benefit of mankind.

in 1747, leonhard euler sent prussian academy of sciences paper in tried prove possibility of correcting both chromatic , spherical aberration of lens. gregory , hall, argued since various humours of human eye combined produce perfect image, should possible suitable combinations of lenses of different refracting media construct perfect telescope objective. adopting hypothetical law of dispersion of differently colored rays of light, proved analytically possibility of constructing achromatic objective composed of lenses of glass , water.

all of euler s efforts produce actual objective of construction fruitless—a failure attributed solely difficulty of procuring lenses worked precisely requisite curves. john dollond agreed accuracy of euler s analysis, disputed hypothesis on grounds purely theoretical assumption: theory opposed results of newton s experiments on refraction of light, , impossible determine physical law analytical reasoning alone.

in 1754, euler sent berlin academy further paper in starting hypothesis light consists of vibrations excited in elastic fluid luminous bodies—and difference of color of light due greater or lesser frequency of these vibrations in given time— deduced previous results. did not doubt accuracy of newton s experiments quoted dollond.

dollond did not reply this, afterwards received abstract of paper swedish mathematician , astronomer, samuel klingenstierna, led him doubt accuracy of results deduced newton on dispersion of refracted light. klingenstierna showed purely geometrical considerations (fully appreciated dollond) results of newton s experiments not brought harmony other universally accepted facts of refraction.

dollond telescope.

as practical man, dollond @ once put doubts test of experiment: confirmed conclusions of klingenstierna, discovered difference far beyond hopes in refractive qualities of different kinds of glass respect divergence of colors, , rapidly led construction of lenses in first chromatic aberration—and afterwards—the spherical aberration corrected.

dollond aware of conditions necessary attainment of achromatism in refracting telescopes, relied on accuracy of experiments made newton. writings show exception of bravado, have arrived sooner @ discovery mind prepared. dollond s paper recounts successive steps arrived @ discovery independently of hall s earlier invention—and logical processes these steps suggested mind.

in 1765 peter dollond (son of john dollond) introduced triple objective, consisted of combination of 2 convex lenses of crown glass concave flint lens between them. made many telescopes of kind.

the difficulty of procuring disks of glass (especially of flint glass) of suitable purity , homogeneity limited diameter , light gathering power of lenses found in achromatic telescope. in vain french academy of sciences offered prizes large perfect disks of optical flint glass.

the difficulties impractical metal mirrors of reflecting telescopes led construction of large refracting telescopes. 1866 refracting telescopes had reached 18 inches (46 cm) in aperture many larger great refractors being built in mid late 19th century. in 1897, refractor reached maximum practical limit in research telescope construction of yerkes observatorys 40-inch (100 cm) refractor (although larger refractor great paris exhibition telescope of 1900 objective of 49.2 inches (1.25 m) diameter temporarily exhibited @ paris 1900 exposition). no larger refractors built because of gravity s effect on lens. since lens can held in place edge, center of large lens sag due gravity, distorting image produces.