118 THE MICROSCOPE. 
divisions, when intended for a stage micrometer as a standard of 
measurement ; but they have no errors that can be recognized when 
used in the ocular as a means of comparing that standard with the 
object, being then magnified only by the power of the eye-lens or of 
the ocular, which seldom exceeds 20 or 30 times. ; 
The exact width of the spaces, if they are quite uniform, is unim- 
portant. There is no positive advantage in having them laid off to 
any even fraction of an inch or of a millimeter, except for the chance 
that they may sometime be required as a stage micrometer; and 
many persons do the best of work with a plate without knowing the 
scale to which it is ruled. 
But the relation of the fineness of the ruling to the power of the 
ocular in which it is to be used, and the resultant apparent distance 
of the lines when in use, determines the reading of the scale and lim- 
its its capacities for positive work. It should therefore be carefully 
considered, and both ruling and ocular be chosen accordingly. If a 
plate ruled to 0.1 mm. (or s4,th inch) be placed in a field of a 1 in. 
ocular, the apparent width of the spaces may be about 5; in., which, 
with a 2 in. objective, may be made to correspond with ;,5,5 in. of 
another micrometer lying on the stage. The ocular scale then reads 
saooths of an inch, and any further nicety depends upon the skill of 
the observer in guessing, with more or less ingenuity, at the halves, 
quarters or fifths of a division; just as a carpenter in using a rule 
divided to eighths of an inch must be satisfied with eighths or guess 
at the sixteenths. An experienced person with a good eye for dis- 
tances, can readily locate any given tenth upon an undivided space 
under easy conditions ; but in the above case he will do well to esti- 
mate accurately fifths of a division, and thus read ;,/>5ths in.* 
The same process with higher objectives would, evidently, give 
proportionately higher readings ; but whatever power be used, only 
one-fifth of the result is positive, the remainder being by guess, and 
that under rather difficult conditions. 
It is therefore advantageous for fine work, to use closer lines, 
and, notwithstanding the writer’s preference for low oculars, to 
transfer the scale to a % or $ in. ocular, which will further separate 
the lines. In this case a ruling at ;1, in. will present lines at a 
convenient distance, and will, of course, read twice as finely as the 
*The writer has repeatedly made trial of a diagonal line, crossing the parallels 
obliquely at lengths increasing regularly by tenths of a unit, after the plan proposed by 
Hartnack ; but he was satisfied that the uncertainty due to a slanting line, which is not at 
right angles to the diameter to be measured nor usually a tangent to the outline of the 
object, introduced danger of error greater than that of estimating with the eye fractions of 
distances between parallel lines. Likewise White’s micrometer, with the glass plate cut 
away from half the field, introduces inequalities of power and definition which seem to 
more than balance its advantages. 
