64 
PROFESSOR W. N. HARTLEY AND HR. W. E. AHENEY 
Normale,’ vol. ix., 1880; and ‘Archives des Sciences Physiques et Naturelles de 
Geneve,’ (3), ii., pp. 119-126). Messrs. Liveing and Dewar, with an improved 
goniometer and a Rutherfurd grating, have estimated the wave-lengths of the lines 
of carbon by a modification of Mascart’s method (Proc. Poy. Society, vol. xxxii., 
1882). As each line must be cussed independently of the rest of the spectrum and 
photographed on a different plate, and as the relative positions of the lines on the 
photographs are varied by very slight alterations in focus and by the removal and 
replacement of the plates, we have been led to apprehend that there are grave objections 
to this method of manipulation. The process, moreover, appears to be a lengthy one. 
In accordance with these views, which are the result of long experience, we have 
preferred to employ a method similar to that of Cornu.* 
As much of each spectrum as possible is photographed on one plate, and together 
with this a series of ideal lines or plain reflections of the slit, each corresponding to a 
measured angular deviation, from which a scale of wave-lengths may be calculated. 
The instruments. — For the production of spectra we have used one of Mr. Ruther- 
furd’s small diffraction gratings. This was mounted on a stand made six years ago 
by Mr. Browning, the telescope and collimator of which are fixed, and the grating 
movable. A tangent screw is used to give an angular motion to the grating, and 
measurements are made upon a divided arc of 9 inches radius. The position of the 
grating can be fixed at any required angle without the necessity of clamping. The 
original telescope and collimator fitted with glass lenses were removed from the stand, 
and replaced by a collimator and two lenses of 36 inches focus for the D lines. The 
material of the lenses was quartz, one of right- the other of left-handed rotation. 
Unless the lenses are approximately of the same thickness and correct one another, all 
fairly strong lines, whether produced by a prism or grating, are liable to be doubled, 
that is to say, accompanied by faint images resembling the “ghosts” that are seen 
when strong lines are viewed with a Rutherfurd grating. The distance of the 
lenses from the grating was about 3 inches. Their considerable focal length gives an 
approximately flat field over a wide range of the spectra. Instead of a telescope, the 
photographic camera described in the Scientific Proceedings of the Royal Dublin 
Society, vol. iii. (“ Description of the Instruments and Processes employed in 
Photographing Ultra-violet Spectra,” 1881, W. N. Hartley) was altered and 
adjusted for use with the grating. The table upon which it was supported consisted 
of a large and massive slab of slate, immovably set upon solid stone foundations. 
In taking photographs from metallic electrodes, it is of some importance that the 
spark be always in exactly the same position with regard to the slit, otherwise the rela¬ 
tive positions of the lines are liable to variation ; we have, therefore, always used an 
electrode of cadmium immovably fixed opposite to the slit, the other points of metal 
* It may be mentioned that the best method for the detennina tion of -wave-lengths, and the precau¬ 
tions to be taken with regard to the accurate measuring of the positions of the lines, was the subject of 
several mouths' investigation by one of the authors, W. N. H. 
