APPENDIX II. 419 



DR. THOMAS YOUNG TO SIR JOSEPH BANKS. 



Worthing, September 10, 1810. 



DEAR SIR, Observing from the papers that you have 

 been interesting yourself respecting the arrangement of a 

 micrometer for the purpose of measuring the diameter of 

 the fibres of different kinds of wool, I beg leave to trouble 

 you with the description of a very simple instrument which 

 I invented some time ago for a similar purpose, and which 

 I propose to call an agricultural micrometer. I should 

 imagine it to be sufficiently accurate for all practical 

 purposes, and the great facility and cheapness of its con- 

 struction may perhaps render it useful to a class of persons 

 who would object to the expense of providing themselves 

 with a more complicated apparatus. If it appear to you in 

 the same point of view, you will be pleased to make any 

 use that you may think proper of this communication. 

 When we look at a distant candle through a lock of wool 

 it appears surrounded by rings of colours, and these rings 

 are invariably so much the larger as the wool is the finer. 

 The cause of these appearances I have endeavoured to 

 explain in my lectures and elsewhere ; but for the present 

 purpose the principal object is to ascertain their com- 

 parative magnitude. In order to perform this I take a 

 card blackened on one side, describe on it two concentric 

 circles, the outer exactly one inch in diameter, the inner 

 li, make at the centre a hole about g 1 ^ of an inch in 

 diameter, and pierce the card at the circumference of the 

 inner circle, in 10 or 12 points at equal distances, with a 

 small pin, and at that of the larger in 7 or 8 points, at 

 unequal distances, with a large one. I then take a small 

 rod of wood about a yard long and divide it into half- 

 inches, numbering them from one end, at which I fix two 

 or three pins side by side and wind round it a piece of 



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