126 THE MICROSCOPE. 
sheep’s blood from “1 to the 9th of “a”; reading respect- 
tively, 27, 23 and 19 one hundred-thousandths of an inch. Thus it 
would be easy to distinguish between all these except the first two, 
and possible in that case if we were certain as to the true averages, 
and sure, which is more than doubtful, that the averages themselves 
may not vary enough to obliterate the narrow margin between them. 
Any one who can subdivide the smallest spaces to tenths, with 
the eye, can, of course, read in millionths of an inch, or in fortieths 
/; but few persons are likely to go, at any advantage, beyond the 
record of the finest lines. These appear wide enough apart to esti- 
mate in fourths or fifths. But this becomes difficult, if not futile, on 
account of diffraction, imperfect definition, inequality in the illumin- 
ation of the scale and of the object, parallax from tremor in both 
apparatus and observer, and error in making optical contact between 
the margin of the object and the line from which measurement is to 
begin; elements which bring a large personal equation into the case, 
as they vary greatly according to the capacity of individual workers 
and the quality of their outfit. 
The above is intended to show what can be done by a skillful | 
person with good, but common-place apparatus. The ruling may 
cost, perhaps, a couple of dollars, and a high-power ocular to carry it 
about twice as much. The objective required for the work is not of 
unusual power or quality ; and any small, plain microscope of fair 
quality and good fine-adjustment, can be employed, a lengthening 
tube being improvised if there be no draw-tube. A screw-movement 
to adjust the lines in the ocular to the image of the object, or else a 
mechanical stage for adjusting the object to them, will be of great 
assistance ; but as the latter, of efficient character and applicable to 
the most unpretending stands, can now be made for $18.00, it is not 
a very unreasonable luxury. 
Having determined with great care the readings of a ruling with 
a certain ocular, objective and tube-length, a record of these data is 
of course preserved, to save time in setting up the apparatus for 
future use; and it is too common to depend entirely on such a 
record, many persons using a scale wholly according to its value as 
once determined by a friend, or by the maker of the microscope. It 
cannot be too strongly urged that the highest precision and safety 
should be secured, at least in all cases of critical importance, as in 
investigations connected with criminal trials, by a careful comparison 
of the scale every time, with a stage micrometer of known quality, 
after the apparatus is set up and ready for use. 
