remarked that he noticed that the eye grew more exact in its demands as it
grew older, in regard to the focal point. A third and very serious
objection to the second method is caused by diffraction from the edges of
the holes or the slit. Let me explain this briefly. When light falls upon
a slit, such as we have here, it is turned out of its course; as the slit
has two edges, and the light that falls on either side is deflected both
right and left, the rays that cross from the right side of the slit toward
the left, and from the left side of the slit toward the right, produce
interference of the wave lengths, and when perfect interference occurs,
dark lines are seen. You can have a very pretty illustration of this by
cutting a fine slit in a card and holding it several inches from the eye,
when the dark lines caused by a total extinction of the light by
interference may be seen.

[Illustration: FIG. 7.]

If now you look toward the edge of a gas or lamp flame; you will see a
series of colored bands, that bring out the phenomenon of partial
interference. This experiment shows the difficulty in obtaining a perfect
focus of the holes or the slit in the diaphragm, as the interference
fringes are always more or less annoying. Notwithstanding these defects of
the two systems I have mentioned, in the hands of the practical workman
they are productive of very good results, and very many excellent surfaces
have been made by their use, and we are not justified in ignoring them,
because they are the stepping stones to lead us on to better ones. In my
early work Dr. Draper suggested a very excellent plan for testing a flat
surface, which I briefly describe. It is a well known truth that, if an
artificial star is placed in the exact center of curvature of a truly
spherical mirror, and an eyepiece be used to examine the image close
beside the source of light, the star will be sharply defined, and will