
Bright Spectrum Lines. 493 
sodium flame or incandescent sodium vapour in a vacuum-tube 
was employed. The slit of the spectroscope being wide 
the two D lines were superimposed so that the two radiations 
together entered the interferometer. The plates were separated 
until the first coincidence happened, and the readings taken; 
the operation was repeated several times. Since the difference, 
da, between the sodium lines is known with accuracy from 
Rowland’s tables, the distance D between the plates can be 
ealeulated from the above equation and thus the zero point 
obtained. Readings were taken of the successive coincidences 
as the plates were separated and in this manner che screw 
was calibrated. Jf a more accurate calibration is required 
the two yellow lines of mercury can be used; since their 
distance apart is about three times that of the D lines, the 
coincidences occur three times more often in a given distance. 
Remarks on Interference-Bands. 
Before considering the results I will add some remarks 
concerning the eeneral character of the interference-bands 
obtained by this interference method. 
When the silvered surfaces are not parallel, but are 
inclined to one another at a small angle, the fringes obtained 
are localised in the plates and, as is well known, can be seen 
by the eye or with a lens focussed on the plates. These 
fringes, however, can only be obtained when the separation 
_ of the plates is very small. 
In order to procure clear interference-bands with great 
differences of paths it is necessary to have the surfaces 
rigidly parallel. The fringes in this case are seen by the 
eye, or by means of a telescope focussed for infinity. One 
of the most important results of this work is that the silvered 
faces of the plates must be perfectly parallel and the telescope 
must be focussed for injinity to obtain correct results. While 
this has been noted by former investigators J wish to 
strongly emphasize the necessity for these adjustments, for if 
these two conditions are not fulfilled all manner of anomalous 
results may be expected. 
On Piate X XV. are shown some photographs of some of the 
results obtained, if these conditions are not obeyed. Figs. 
6-12 were all taken with the bright green line of incandescent 
mereury vapour in a vacuum- -tube. None of the photographs 
are magnified, the focal length of the objective used was 
about 15 ems. 
The separation of the interference-plates in figs. 1, 6, 
and 7 was mms. 1 is where the adjustments are per fect, 
