seen in telescope _a_. The two telescopes are now mounted as in Fig. 2,
and the plate to be tested placed in front of the two telescopes as at
_c_. It is evident, as in the former case, that if the surface is a true
plane, the reflected image of the holes or slit thrown upon it by the
telescope, _b_, will be seen sharply defined in the telescope, _a_.

[Illustration: FIG. 5.]

If any error of convexity exists in the plate, the focal plane is
disturbed, and the eyepiece must be moved _out_. If the plate is concave,
it must be moved _in_ to obtain a sharp image. Irregular errors in the
plate or surface will produce a blurred or indistinct image, and, as in
the first instance, no amount of focusing will help matters. These methods
are both good, but are not satisfactory in the highest degree, and two or
three important factors bar the way to the very best results. One is that
the aberrations of the telescopes must be perfectly corrected, a very
difficult matter of itself, and requiring the highest skill of the
optician. Another, the fact that the human eye will accommodate itself to
small distances when setting the focus of the observing telescope. I have
frequently made experiments to find out how much this accommodation was in
my own case, and found it to amount to as much as 1/40 of an inch. This is
no doubt partly the fault of the telescopes themselves, but unless the eye
is rigorously educated in this work, it is apt to accommodate itself to a
small amount, and will invariably do so if there is a preconceived notion
or bias _in the direction of the accommodation_.

[Illustration: FIG. 6.]

Talking with Prof. C.A. Young a few months since on this subject, he
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