may now ask, how are we to know what sort of surface we have? A ready
solution is at hand. The bands _always travel in the direction of the
thickest film of air_, hence on lowering the eye, if the convex edge of
the bands travel in the direction of the arrow, we are absolutely certain
that that part of the surface being tested is convex, while if, as in the
central part of the bands, the concave edges advance, we know that part is
hollow or too low. Furthermore, any small error will be rigorously
detected, with astonishing clearness, and one of the grandest qualities of
this test is the absence of "personal equation;" for, given a perfect test
plate, _it won't lie_, neither will it exaggerate. I say, won't lie, but I
must guard this by saying that the plates must coincide absolutely in
temperature, and the touch of the finger, the heat of the hand, or any
disturbance whatever will vitiate the results of this lovely process; but
more of that at a future time. If our surface is plane to within a short
distance of the edge, and is there overcorrected, or convex, the test
shows it, as in Fig. 10. If the whole surface is regularly convex, then
concentric rings of a breadth determined by the approach to a perfect
plane are seen. If concave, a similar phenomenon is exhibited, except in
the case of the convex, the broader rings are near the center, while in
the concave they are nearer the edge. In lowering the eye while observing
the plates, the rings of the convex plate will advance outward, those of
the concave inward. It may be asked by the mechanician, Can this method be
used for testing our surface plates? I answer that I have found the
scraped surface of iron bright enough to test by sodium light. My
assistant in the machine work scraped three 8 inch plates that were tested
by this method and found to be very excellent, though it must be evident
that a single cut of the scraper would change the spot over which it
passed so much as to entirely change the appearance there, but I found I
could use the test to get the general outline of the surface under process
of correction. These iron plates, I would say, are simply used for