34 = do. M. Blake—WNotes on Diffraction Gratings. 
When a ruled plate is laid upon a film and lighted by a 
window, a series of rings or irregular bands can be seen on the 
grating, and similar ——— are reproduced in the photo- 
h. These were at first thought to be Newton’s rings. 
They varied on pressure, but could not be iad to disappear 
by this means in every case; as the slightest irregularity of . 
the film prevented Fasleae: contact. It was found that a 
separation of a thickness of paper between the surfaces did not 
cause these bands to disappear, but increased their number. 
They were nar aap not due to the “colors of thin plates.” 
It will readily occur that these bands are of the same character 
as the effect seen on viewing one picket fence through another 
or the shadow of a piece of wire cloth through the cloth itself. 
The two sets of lines coincide in the light spaces, while in the 
dark bands the lines of the two series alternate. In the case 
we have to deal with there are one or more reflections between 
the brilliant surface of the film and the ruling. The precaution 
was always taken to stop reflection from the back of the sensi- 
tive plate by backing in optical contact. It may be interesting 
to note in this connection that a series of spurious bands can 
be seen crossing the spectra of a lamp when looking in one 
oblique direction through a glass ruling, the ruled surface 
being next the eye. When this position is found, the bands 
can be made to cross the first and second spectra, either 
parallel to true spectral lines, or obliquely, by rotating the 
glass plate in its own plane. These bands are evidently of the 
same character as mee me have been described. 
we take a Ts h which was ate with imperfect 
contact, and hold it aie from the eye toward a eas so as 
e 
not the two colors combined, as would be the case in a perfect 
grating. In some photographs the contact has been so perfect 
that no dark bands appear until the third or fourth apeotrie is 
reached. The first spectrum is little impaired in any of the 
photographs of the 6480 line grating. The most brilliant spec- 
trum observed was through a ang which showed numerous 
dark bauds 4 in the second and third spectra. It is believed 
that, notwithstanding the defect causing these bands, there is 
or > gs Tee MR 
