1879. | Microscopy. 339 
in'charge by which it could be made practically useful to the 
microscopists of the country. If such a standard does not exist, 
one should be made; or if it is not within the power of our pres- 
ent art to give results in which the microscope cannot find a fault, 
then the end could be attained, measurably well, by selecting a 
standard as perfect as era and attaching to it a statement of 
its carefully determined erro 
his standard, if it be salar to call it a standard at all, should 
be selected by, or made for, the committee, should be the e prop- 
erty of the nation, and should have no mercaitile relations what- 
ever. By means of a limited number of very carefully verified 
copies, which could also be used as a means of reconstruction in 
case of accidental destruction of the original, it could be rendered 
available to all persons who make or use micrometers. The 
cheapest commercial scales, even, could without increase of price 
be accompanied by a statement as to how closely the work of the 
screw which ruled them has been found to correspond with the 
standard; and all plates claiming a high degree of accuracy 
could be carefully compared, space for space, and accompanied by 
a statement of the ascertained error of each individual space. 
Persons of really scientific training would gladly incur the extra 
some cases, receive additional value by a note, added in later edi- 
tions, stating how nearly the apparatus used by the author has 
been ‘found to correspond with the standard afterwards adopted. 
Of course this standard would deserve the name only in a limi- 
ted sense, and not in the same sense as the standard yard in Lon- 
don and the standard metre in Paris; but it would be an offi- 
cially recognized representative of some unit practicable in micro- 
scopy, and it might be made to add greatly to the uniformity and 
value of our work. 
The ate of which the standard should be made, the form 
it should take, the manner in which the spaces should be sete 
cated, the temperature at which it should be standard, and t 
manner in which it should be made available to the public, at ex in 
which its safety could be best secured, could only be determined 
after careful consideration of the world’s recent experience in re- 
Spect to the care and use of precise ea of length. Te has 
controlled, the adoption of a screw, however accurate it might be, 
would be ‘likely to result in the ‘dissemination of a number of 
tuled plates possessing equal authority as standards, but differing 
measurably from each other, Whether the micrometric stand 
should be taken from our national standard, at Washington, 
