This grating is believed to have been originally owned by Alvan Clark & Sons of Cambridgeport, Massachusetts: the best American telescope-makers of the nineteenth century. It is now owned by John W. Briggs.† The grating is signed “Lewis M. Rutherfurd”, is dated “1875”, and is marked with the number of ruled lines per inch, 5,761, and total number of lines, 8,649.
An electron micrograph of the 1875 grating shows a substrate (speculum metal) that is scratched. It was insufficiently polished prior to being ruled, but it is also marred by prominent scratches made after ruling. The grooves are shallow and narrow relative to the spacing, and show variations in both width (≈0.70 µm with ≈5–7% variation) and interline spacing of about 2.75 ± 0.15 µm. The flaws in the diamond point are clear, having left three fine lines within each groove. The diamond was also throwing up a burr on each side of the groove. This ruling was executed from left to right, as shown by the effect of the underlying scratches on the burrs.
High magnification electronmicrograph of the Rutherfurd grating.
[†Access to the Rutherfurd grating was provided courtesy of John W. Briggs, Yerkes Observatory. This and the following electronmicrographs were provided by the Canadian Conservation Institute.]
