354 
NOTES AND MEMORANDA. 
used, there can be no doubt, and those who are about to provide 
themselves with an entirely new set of standards would do well to 
adopt for general work the metre or one of its subdivisions, but our 
power to reduce inches to millimetres by calculation (1 mm. = 
0*03937043 inch) greatly exceeds in delicacy our power to make 
observations, and where any observer has already employed the inch 
or is provided with standards which he has carefully examined and 
compared, it becomes a serious question whether or not he should make 
a change. 
It may be well, however, to bear in mind that it is not more than 
once in a hundred times that micrometric measurements are stated in 
absolute quantities, that is, in terms of the inch or metre. In most 
cases the micrometer is used merely to ascertain the extent to which 
the object is magnified, and this is generally effected by placing a 
micrometer on the stage, and making a drawing of its divisions under 
the same power that the object itself is drawn. The drawing of the 
enlarged image of the micrometer then becomes a scale for measure- 
ment, its absolute value being determined by applying to it an 
ordinary rule or measure. For this purj)ose the particular standard 
that is used is of no consequence whatever, but so long as foot rules 
and scales divided into inches are more common than rules divided 
into millimetres, just so long will micrometers expressing divisions 
of the inch be more generally convenient than micrometers divided 
after the metric system. The measurements of blood-corpuscles are 
perhaps the only absolute measurements generally made, and in the 
literature of this subject (at least, that in the English language, 
which, by the way, is the best literature relating to it) the divisions 
of the inch are always used. Those who propose to investigate 
this subject, will save themselves much useless labour by employing 
a micrometer having divisions of the inch. 
The proposition to use the so-called metric standard in preference 
to the inch and its subdivisions may therefore be dismissed as one 
having little power for good or ill. As the metric system comes into 
use generally, so it will come into use amongst microscopists, and not 
till then. Every scientific man will, of course, hail its adoption with 
pleasure. 
The proposal to use the of a millimetre as the new unit is, 
however, of a much more mischievous tendency. It puzzles us to see 
to what possible advantage it can give rise, or why it should have 
been brought forward at this day, after having been proposed by 
Harting years ago, and justly consigned to oblivion by all intelligent 
workers. The principal reasons for and against the adoption of a y^y 
of a millimetre as a so-called unit are as follows ; — 
In favour of it we have the avoidance of fractions, of which the 
following may be taken as an illustration The yA^ of a millimetre 
is about the 2 sVir ’ instead therefore of speaking of the 
of an inch we might use the term “ three units,” which at first 
seems more simple and easy of expression. In fact, however, it is not 
so. A “ quarter of a yard ” is an expression quite as easily com- 
