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 established the laws of the appearances presented. I will now endeavor
to explain the general principle by which this phenomenon is utilized in
the testing of plane surfaces. Suppose that we place on the lower plate,
lenses of constantly increasing curvature until that curvature becomes
nil, or in other words a true plane. The rings of color will constantly
increase in width as the curvature of the lens increases, until at last
one color alone is seen over the whole surface, provided, however, the
same angle of observation be maintained, and provided further that the
film of air between the glasses is of absolutely the same relative
thickness throughout. I say the film of air, for I presume that it would
be utterly impossible to exclude particles of dust so that absolute