d into two
hundred parts, and by estimation can be read to 1/100000 of an inch. Its
constants are known, and it may be understood that it would not do to
handle it very roughly. I could dwell here longer on this fascinating
subject, but must haste. I may add that if this spherometer is placed on a
plate of glass and exact contact obtained, and then removed, and the hand
held over the plate without touching it, the difference in the temperature
of the glass and that of the hand would be sufficient to distort the
surface enough to be readily recognized by the spherometer when replaced.
Any one desiring to investigate this subject further will find it fully
discussed in that splendid series of papers by Dr. Alfred Mayer on the
minute measurements of modern science published in SCIENTIFIC AMERICAN
SUPPLEMENTS, to which I was indebted years ago for most valuable
information, as well as to most encouraging words from Prof. Thurston,
whom you all so well and favorably know. I now invite your attention to
the method for testing the flat surfaces on which Prof. Rowland rules the
beautiful diffraction gratings now so well known over the scientific
world, as also other plane surfaces for heliostats, etc., etc. I am now
approaching the border land of what may be called the abstruse in science,
in which I humbly acknowledge it would take a vast volume to contain all I
don't know; yet I hope to make plain to you this most beautiful and
accurate method, and for fear I may forget to give due credit, I will say
that I am indebted to Dr. Hastings for it, with whom it was an original
discovery, though he told me he afterward found it had been in use by
Steinheil, the celebrated optician of Munich. The principle was discovered
by the immortal Newton, and it shows how much can be made of the ordinary
phenomena seen in our every-day life when placed in the hands of the
investigator. We have all seen the beautiful play of colors on the soap
bubble, or when the drop of oil spreads over the surface of the water.
Place a lens of long curvature on a piece of plane polished glass, and,
looking at it obliquely, a black central spot is seen with rings of
various width and color surrounding it. If the lens is a true curve, and
the glass beneath it a true plane, these rings of color will be perfectly
concentric and arranged in regular decreasing intervals. This apparatus is
known as Newton's color glass, because he not only measured the phenomena,
but e