384 On the Fibres used in Micrometers, &c. 



which enables us to remove the error of inflection, while it 

 possesses the requisite properties of opacity and elasticity. 

 This fibre is made of glass, which is so exceedingly elastic, 

 that it can be drawn to any degree of fineness, and can al- 

 ways be procured and prepared with facility; a circumstance 

 of no small importance to the practical astronomer, who is 

 frequently obliged to send his micrometers to a great distance 

 to be repaired. 



It is evident that this vitreous fibre, when drawn from a 

 hollow glass tube, will also be of a tubular structure, and 

 that its interior diameter may always be regulated by the 

 inner diameter of the original tube. When the fibre is 

 formed, and stretched across the diaphragm of the eye-piece 

 of a telescope, it will appear perfectly opaque, with a deli- 

 cate line of light extending along its axis. This central 

 transparency arises from the transmission of the incident 

 light through the axis of the hollow tube : and since this 

 tube can be made of any calibre, we can also increase or di- 

 minish the diameter of the luminous streak. In a micro- 

 meter which I have fitted up in this way, the glass fibres 

 are about the 1200dth part of an inch in diameter; and the 

 fringe of light which stretches across their axis is distinctly 

 visible, though it does not exceed the 3000dth part of an inch. 



In using these fibres for measuring the angle subtended by 

 two luminous points, whether they be two stars, or the op- 

 posite extremities of a luminous disc, we may, as has been 

 done hitherto, separate the fibres till the luminous points 

 are in contact with their interior surfaces ; but in order to 

 avoid the error arising from inflection, I would propose to 

 separate the fibres till the rays of light issuing from the lu- 

 minous points dart through the transparent axes of the fibres. 

 The rays thus transmitted evidently suffer no inflection in 

 passing through the fibre to the eye ; and besides this ad- 

 vantage we have the benefit of a delicate line about one third 

 of the diameter of the fibre itself. 



I am, dear sir, your most obedient servant, 



D. Brewster. 

 To Mr. Tilloclu 



LXVIII. Oh- 



