38S SURVEY OF THE COAST 



what chance introduces. 1 hose, however, about thhteeu 

 indies long, for some of the better geodeslcal instruments, 

 and six inches long for reflecting instruments, are the most 

 careliilly attended to. Above the three feet and a half tele- 

 scope, those of five feet focal length, and three to four inches 

 aperture, are frequently obtained very good ; and Mr. Tully. 

 Optician in Islington, has been very successful in them. 

 Those of six feet focal length are difficult to obtain in great 

 perfection. They have the same aperture as the five feet 

 ones ; and Mr. DoUond has made some very good telescopes 

 of this size, though of course after many trials, and the re- 

 jection of many glasses. 



A good telescope of seven feet focal length and greater, 

 with four to five inches aperture, may be considered as the 

 result of a happy chance, and proportionally to what is ex- 

 pected of such telescopes, I heard only of about three or 

 four that had acquired a well deserved fame. Of these the 

 largest and best is the ten feet telescope, with six inches 

 aperture, made by old Mr. Dollond for the Greenwich Ob- 

 servatory, to prove the possibility of making an achromatic 

 telescope equally as good as the ten feet reflecting telescope 

 of Dr. Herschel. Tn this he actually succeeded, but only in 

 one glass out of a number made with that view. This tele- 

 scope has lately been adapted by Mr. Troughton to a transit 

 instrument for the Observatory. 



The sizes intermediate between five feet and twenty -two 

 inches are considered as generally speaking not good. For 

 instance, the four feet focal length is not suitable for scien- 

 tific purposes. They are therefore generally mounted for 

 pocket telescopes or spy-glasses. 



In the larger telescopes, the optician is not quite certain 

 of the focal length in the formation of his object glasses. 

 This may vary within two inches, though tiie glasses come 

 out of the same forms: but such a difference is of no impor- 

 tance. 



A high polish to the object glasses is not desirable in in- 

 struments. The glaring liglU which it occasions is not agree- 



