114 Mr. Troughton on dividing Instruments . 
divide the whole by hand at once, and afterwards to correct 
the whole ; for a dot forced to its place, as above, will seldom 
allow the compass-point to rest in the centre of its apparent 
area ; therefore other dots made from those will scarcely ever 
be found in their true places. This improvement also pre- 
vents the corrected dots from being injured, or moved, by 
the future application of the compasses, no such application 
being necessary. 
I will now dismiss this method of dividing, with observing, 
that it is tedious in the extreme ; and did I not know the con- 
trary beyond a doubt, I should have supposed it to have sur- 
passed the utmost limit of human patience.* When I made 
my first essay at subdividing with the roller, I used this me- 
thod, according to the improvement suggested above, of cor- 
recting a few primitive points ; but even this was too slow for 
one who had too much to do. Perhaps, however, had my 
instruments been divided for me by an assistant, I might not 
have grudged to have paid him for the labour of going through 
the whole work by the method of adjustment ; nor have felt 
the necessity of contriving a better way. 
I might now extend the account of my method of divi- 
ding to a great length ; by relating the alterations which the 
* At the time alluded to, the double microscopic micrometer was unknown to me, 
and I did not learn its use, for these purposes, till the year 1790, from General Roy’s 
description of the large theodolite. Previous to that time, I had used a frame which 
carried a single wire very near the surface to be divided ; this wire was moveable by 
a fine micrometer screw, and was viewed by a single lens inserted in the lower end of 
a tube, which, for the purpose of taking off the parallax, was 4 inches long. The 
greatest objection to this mode of constructing the apparatus is, that the wire being 
necessarily exposed, is apt to gather up the dust ; yet it is preferable to the one now 
in use, in cases where any doubt is entertained of the accuracy of the plane which is 
to receive the divisions. 
