170 L Bell—Absolute Wavelength of Light. 
on an adjustable platform with a circle 12°5™ in diameter, 
divided to 30’ by verniers to 1’ and moving either upon or with 
the large circle. 
This arrangement of parts does not admit of fixing the grat- 
ing rigidly normal to the collimator, so in all the experiments 
it was placed normal to the observing telescope, a position 
which was particularly advantageous in the matter of adjust- 
ment. The instrument was set up in a southern room in the 
physcial laboratory and throughout the experiments the colli- 
mator pointed about south-southeast. With the eyepiece used 
the observing telescope had a power of very nearly sixteen 
diameters. d 
GRATINGS. 
Very few glass gratings have ever been ruled on Prof. Row- 
land’s engine, since for most purposes they are much inferior to 
the metallic ones, and are very much more difficult to rule, as 
they run great risk of being spoiled by the breaking down 
of the diamond point. A very few, however, were ruled in 1884 
with special reference to wave-length determination, and of 
these the two best were available for these experiments. They 
are both ruled upon plane sextant mirrors, and are of very 
nearly the same size—thirty millimeters long, with lines of 
about nineteen millimeters. Hach hundredth line is longer, and 
each fiftieth line shorter than the rest, so that the gratings are 
very easy to examine in detail. The ruling of both is smooth 
and firm, without breaks or accidental irregularities and almost 
without flaws. They were ruled at different temperatures 
and on different parts of the screw, and while one was ruled 
with the ordinary arrangement of the engine, the other 
was ruled to a very different space by means of a tangent 
screw. This great diversity of conditions in the two gratings is 
far from favoring a close agreement in the results, but tends to 
eliminate constant errors due to the dividing engine, and hence 
to increase the value of the average result. It must be remem- 
bered that two gratings ruled on the same part of the screw are 
in most respects little better than one. The grating designated 
I in this paper contains 12,100 spaces, at the rate of very nearly 
400 to the millimeter, and was ruled (by tangent screw) at 
a temperature of 6°°7 C. in January, 1884. It gives excellent 
definition with almost exactly the same focus for the spectra on 
either side, and is quite free from ghosts or other similar 
defects. 
The grating designated II has 8,600 spaces, at the rate of 
about 7,200 to the inch, and was ruled in November, 1884, at 
11°-6C. Its definition and focussing are very nearly as good as 
in I and like it, it shows no trace of ghosts or false lines; they 
