ON DIVIDING ASTRONOMICAL INSTRUMENTS. 305 



that by means of its a^ljustable a>c, it makes the run of tlie 

 roller measure its correspondino^ intervals upon the circle; 

 and, without foreign aid, furnishes the means of reducing 

 the bisectioaal intervals to the usual division of the circle. 

 Farthermore, the motion of the wire of the micrometer H, 

 accordini^ to the divisio.i of its head and corresponding table 

 of errours, furnishes the means of prosecuting the work 

 with nearly the same certainty of success, as could have 

 I)ap>)ened, had the 256 points been (which in practice is 

 quite impossible) in their true places. 



Now the whole of ray method of dividing being per- Why term e<l 

 formed by taking short measures with instruments which '^''^"^'"S t>y 

 cannot themselves err in any sensible degree, and, inas- 

 much us those measures are taken, not by the hand, but 

 hy vision, -and the whole performed by only looking at the 

 work, the eye must be charged with all the errours that 

 are committed, until we come to cut the divisions ; and, as 

 in this last operation the hard has no more to do than to 

 guide an apparatus so perfect in itself, that it cannot be 

 easily made to deviate from its proper course, I would wish 

 to distinguish it from the other methods by denominating 



it, DIVIDING BY THE EYE*. 



The 



* I must here remark, that Smeaton has represented the greatest de- Degree of ac- 

 gree of accuracy that can be derived from vision, in judging of the coin- curacy of the 

 cidcnce of two Jines, at ^-J^.,;- part of an inch. From this it may fairly ^'S^* ^ touch. 

 be inferred, that he had net eultivated the power of the sight, as he had 

 done that of the touch j the latter of which, with that ability which appear- 

 ed in all his works, he rendered sensible to the __l._^ part of an inch. 

 Were materials infinitely hard, no bounds could be set to the precision of 

 contact; but taking things as they are, the different degrees of hardness 

 in matter, may be considered as a kind of mapnifjing power to the touch, 

 which may not unaptly be compared with the assistance which the eye 

 receives from glasses. It is now quite common to divide the seaman's 

 sextant to 10", and a good eye will estimate the half of it; which, on 

 an eight inch radius is scarcely -j-^^-^-^ of an inch This quantity, small 

 as it is, is fs-ndered visible by a glass of one inch focal length ; and such 

 is the cei taint j' with which these quantities are seen, thataseaman will 

 sometimes coiiii>lain that two pair of these lines will coi;icidcat the same 

 time; and this may happen, and yet n© division of his instrument err 

 by m('re than ^^^^^ part of an inch. All this is apvjlicabla to judging of 

 the coincidence of lines with each other, and furniskes n»t the most fa- 

 vourable 



