micrometer eye-piece used was substantially the same as Foucault's, except
that part of the inclined plate of glass was silvered, thus securing a
much greater quantity of light. The deflection having reached 43
millimeters, the inclined plate of glass could be dispensed with, the
light going past the observer's head through the slit, and returning 43
millimeters to the left of the slit, where it could be easily observed.

Thus the micrometer eye-piece is much simplified, and many possible
sources of error are removed.

The field was quite limited, the diameter being, in fact, but little
greater than the width of the slit. This would have proved a most serious
objection to the new arrangement. With the new lens, however, this
difficulty disappeared, the field being about twenty times the width of
the slit. It was expected that, with the new lens, the image would be less
distinct; but the difference, if any, was small, and was fully compensated
by the greater size of the field.

The first observation with the new lens was made January 30, 1879. The
deflection was 70 millimeters. The image was sufficiently bright to be
observed without the slightest effort. The first observation with the new
micrometer eye-piece was made April 2, the deflection being 115
millimeters.

The first of the final series of observations was made on June 5. All the
observations previous to this, thirty sets in all, were rejected. After
this time, no set of observations nor any single observation was omitted.