On the Fibres used in Micrometers, &c. 323 



way and the bottom of the river between D and E is no 

 where less than 28 feet, and from D to A no where 1cj» 

 than 25 feet. 



The parts shaded in plan Fig. 2 are buildings. 



LXVII. On the Fibres used in Micrometers: With an Account 

 of a Method of removing the Error arising from the hl- 

 flect ion of Light, by employing Hollow Fibres of Glass, 

 By David Brewster, LL. D., Fellow of the Royal 

 Society of Edinburgh, and of the Society of the Anti- 

 quaries of Scotland. 



H 



DEAR SIR, 



aving directed my attention for some years to the con- 

 struction of micrometers, I have had frequently occasion to 

 regret the difficulty of procuring fibres sufficiently fine and 

 elastic for these delicate instruments. The impossibility of 

 obtaining silver wire of a diameter small enough for this 

 purpose, has induced Mr. Troughton to use the web of the 

 spider, which he has found so fine, opaque, and elastic, as 

 to answer all the objects of practical astronomy. I am in- 

 formed, however, by this celebrated artist, that it is only 

 the stretcher or the long line which supports the spider's 

 web, that possesses these valuable properties. The other 

 parts of the web, though equally fine and elastic, are very 

 transparent, and therefore completely unfit for micrornetri- 

 cal fibres. The difficulty of procuring the proper part of the 

 spider's web, has compelled many opticians and practical 

 astronomers to employ the raw fibres of un wrought silk, or, 

 what is much worse, the coarse silver wire which is manu- 

 factured in this country. But whatever be the relative ad- 

 vantages of these different substances, they are all liable to 

 the error arising; from the inflection of light, which renders 

 it impossible to ascertain the exact contact between the fibre 

 and the luminous body. This disadvantage lias been ex- 

 perienced by every astronomical observer, and has always 

 been considered as inseparable from the wire micrometer. 

 I have, however, succeeded in ^obtaining a delicate fibre 



which 



