94 
Farrants, on a Micrometer Object-finder. 
position as at first ; the accuracy of the register can thus be at 
once ascertained,, or any error be detected and corrected. 
As may be expected, it in fact seldom happens that any 
line on the ruled slip will exactly coincide with the position 
of an object, which is more frequently found to lie in the 
interval between two lines. It is better therefore to make 
the register refer to the spaces instead of to the lines. The 
number 45 then, for example, denoting the latitude of an 
object, will indicate that its position is in the space expressed 
by that number. 
To register the position of an object it is only necessary to 
put down in figures the numbers which denote the spaces 
observed ; for instance, the position of the object just supposed 
to have been ascertained would be registered thus : 
Lat. 45. Long. 40. 
It will save trouble if an arbitrary significance be given to 
position. Thus it may be determined that the first number 
shall always refer to latitude, in which case the register might 
stand thus : 
If or 45 : 40. 
I prefer the latter, because the figures better admit of any 
appendix which may be used to modify their significance. 
For example, the object whose position is found to correspond 
with the square denoted by 45 : 40 may lie nearer to the 
upper line than to the lower one, or the contrary ; now this 
may be clearly indicated by a short line added to the figures. 
Thus, if the object lie nearer the upper line, that may be 
indicated by a short line above the figures, 45 ; if the object 
lie nearer the lower line the fact may be indicated by a line 
below the figures thus, 45. Similarly the greater proximity 
of an object to the vertical line on the right or left of it may 
be shown by a short line before or after the figures, as |40 40|, 
the meaning of the line so placed is obvious. Again, 
if the position of the object coincide exactly with a line on 
the ruled slip, that may be denoted by drawing a line through 
the figures thus, 4& ; if then the position of any object be 
registered thus, 4& : |40, it will signify that its latitude corre- 
sponds with the 45 th horizontal line of the ruled slip, while its 
longitude corresponds with the 40th vertical space, but that it 
is nearer to the line on the left side. In this way then four 
positions in respect of each square may be clearly pointed out, 
and the area of each square being the ToioTy'^h of ^ square 
inch, it follows that the place of an object can be assigned 
with an exactness nearly approaching the 4 o^o o^^ of ^ square 
inch. As the field of view with the i-inch object-glass with 
