« 8o Prcpi/Jfir aadlng j« Irli tc 'Tekfcc^ts. 



XII. 



Vfejul Nc.'icfj TtfpeSlitig vaiitus OhjtSfj. — Inprovenietit of Telejcofis. — ImperJeHtons of Optical 



Ghfs, — Purification of Mercury 



J. HE eye haf often been defcribed with admiration by opticians as a perfccfl inftrumcnr, 

 as well as by others in a more loofe way, in illuftvatlon of the argument from final caufes. 

 When t!ie correction of the aberrations from fphericity and colour began to be invefti- 

 gated, it was prefumed that the arrangement and figure of its parts were directed to re- 

 medy thefe defe£is ; a notion which has fince been fufBciently refuted by Drs. Maflcelyne* 

 and Blair f. If the general remarks had ever been extended to the improvement of opti- 

 cal inftrumcnts, inftead of having been applied to ihem fubfequent to the periods of ac\ua! 

 difcovcry ; it might afford matter for furprife, that the mod variable of all its adjuftments, 

 •namely, that of aperture, has never yet been introduced into our artificial combinations. 

 When we look towards a window or any luminous obje£l, the iris immediately contra£ts ; 

 and on the contrary, when wc direft our fight to the inner part of a room, or to any obfcurc 

 place, the aperture for the reception of light is as fpeedily enlarged. To what extent this 

 enlargement may be carried in fituations of extreme darknefs, cannot be obferved ; but it is 

 certain, that in the obfervable variations of the human eye, the aperture is thirty times as 

 large at one time as at another ; and that in the cat the proportion is much greater than 

 an hundred to one. Hence we might reafonably infer, on general principles, that the dif- 

 tlnftnefs of vifion through telefcopes would be improved, if this adjuftment alfo were 

 adi'ed to that of mere focal diftance. 



Every attentive obferver mud have taken notice, that light is of as much confequence to 

 artificial vifion as magnifying power. Diftant woods and other land objefts are invifible to 

 an high magnifying power for want of light, when the fame objefts may be diftinftly 

 feen with a lower. Luminous obje£\s are feen very perfeftly by a fmall aperture, though 

 coloured by a greater : but objefts lefs enlightened will receive all the advantage of fuperior 

 diftindlnefs from greater light, when the fringes of colour may be too faint to be of any 

 confequence. From thefe circumftances it frequently happens, that an achromatic perfpec- 

 tive, admitting plenty of light, may exhibit land obje(Sts, efpecially in the evening, to much 

 greater advantage than a refleCtor, admitting lefs light but performing its office with more 

 accuracy ; whereas, on the other hand, the latter inftrument fhall have greatly the advan- 

 tage when tried upon a planet. 



An ingenious mechanic will find little difficulty in contriving an artificial iris. Suppofe a 

 brafs ring tofurround the obje£i: end of the telefcope, and upon this let eight or more trian- 

 gular flips of brafs be fixed, fo as to revolve on equi-difl.3nt pins palDng through each triangle 

 near one of its corners. If the triangles be Aided inw.rds upon each other, it may readily 

 be apprehended that they will clofe the aperture ; and if they be all made to revolve or Aide 

 backwards alike, it. is clear that their edges will leave an o£lagonal aperture, greater or lefs 

 according to circumftances. The equable motion of all the triangles may be produced 

 either by pinions and one concave toothed wheel, or by what is called fnail-work. Another 

 kind of iris more compafl may be made by cauGng thin elaftic flips of brafs to flidc along 



* Philof. Trinf. LXXIX. Art. ii, + Edinburgh Tianf. II. 



parallel 



