m 
Mr. Troughton on dividing Instruments . 
hand 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 number of persons at all capable of dividing originally 
have hitherto been very few ; the practice of it being so 
limited, that, in less than twice seven years, a man could 
hardly hope to become a workman in this most difficult art. 
How far I shall be considered as having surmounted these 
difficulties, I know not ; but if, by the method here revealed. 
* I must here remark, that Sueaton has represented the greatest degree of accu- 
racy that can be derived from vision, in judging of the coincidence of two lines at ^ a 8 a y , 
part of an inch. From this it may fairly be inferred, that he had not cultivated the 
power of the sight, as he had done that of the touch ; the latter of which, with that 
ability which appeared in all his works, he rendered sensible to the part 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 magnifying power to the touch, which may not un- 
aptly be compared with the assistance which the eye receives from glasses. It is now 
quite common lo 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 of an inch. This quantity, 
small as it is, is rendered visible by a glass of one inch focal length ; and such is the 
certainty with which these quantities are seen, that a seaman will sometimes complain 
that two pair of these, lines will coincide at the same time ; and that may happen, and 
yet t o division of his instrument err, by more than part of an inch. All this 
is applicable to judging of the coincidence of lines with each other, and furnishes not 
the most favourable display of the accuracy of vision. But with the microscopes here 
described, where the wire bisects the image of a dot, or a cross wire is made to in- 
tersect the image of a line, by an eye practised in such matters, a coincidence may 
undoubtedly e ascertained to rEn hny part of an inch. I am of opinion that as small 
a quantity may be rendered visible to the eye, as can by contact be made sensible to 
the touch ; but whether Mr. Smeaton’s and my be not the same thing, 
I will not determine; the difference between them, however, is what he would no 
more have pretended to feel, than I dare pretend to sec. 
