EDUCATIONAL VALUE OF EXACT MEASUREMENT. 559
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quently furnished Newton the data on which to frame his theory of the
lunar motions and to show that they were conformable to his law of gravi-
tation. Also, by measures made with the micrometer eyepiece, Flam-
steed gave Newton and Halley the data which enabled them to compute
the orbit of the comet of 1680.
The micrometer screw was subsequently used by Ramsden in his
machines for dividing the straight line and the circle. Ramsden’s inven-
tion, improved by our distinguished countrymen Rutherfurd and Row-
land, has in their hands done noble work in ruling diffraction plates with
great precision, and in giving the most exact measures yet made of the
wave-lengths of light.
The vernier I need not describe. It is familiar to all educated persons.
Suffice it to say that its immediate adoption as a means of subdividing
the smallest divisions of a scale, either linear or circular, introduced a
degree of precision in measurement not thought of before the middle of
the seventeenth century.
Precise science is not older than two hundred and fifty years, only five
times the length of the fifty years which to one of the age of sixty is looked
back upon as a short period, in which but little can be accomplished by
‘ndividual effort.
The effects on the formation of modern science of the two notable inven-
tions I have referred to are undeniable, and it will be noted by all students
bf physical science that in every addition to our means of exact measure-
ment, whether of geometric magnitudes or of the units used in light,
heat, or electricity, sudden advances are made in knowledge ; for to meas-
ure the connected parts of phenomena is the first and indispensable step
to the advance to the knowledge of the Jaw which connects the associated
facts of the phenomena.
Such advances are recalled by all of you. After the invention of the
revolving mirror by our countryman, Joseph Saxton, in 1830, it was
applied by Wheatstone in 1834 to the measurement of the velocity of
slectricity ; and its subsequent use by Arago, Foucault, and Michelson to
exact measurements of the velocity of light marks a notable progress in
measures of precision. Rood, of New York, using the same invention,
succeeded In measuring precisely a fraction of time as small as the
rsooooss Of a second.
Similar advances in our knowledge of the radiant energy of the sun,
moon, and stars, and of the distribution of heat in the solar spectrum, fol-
lowed the invention of the holometer in 1881 by Professor Langley, the
present secretary of the Smithsonian Institution. To enumerate all the
advances due to the invention of new instruments for exact measurements
would be to give a good part of the history of modern physics and
astronomy.
If exact measurements, as I have attempted to show, permeate all the
