538 
ME. W. HUGGINS ON THE SPECTRA 
it was proposed to seek for in this investigation. The observations of many nights have 
been rejected, from the uncertainty as to the possible existence of an accidental displace- 
ment. 
Another inconvenience, so great as even to seem to diminish the hope of ultimate suc- 
cess, was found to arise from the difficulty of bringing the lower margin of the star-spec- 
trum into actual contact with the upper margin of the spectrum of the light reflected 
into the instrument. The lines in the spectra of the stars are not, on ordinary nights, 
so steady and distinct as are those of the solar spectrum. Under these difficult circum- 
stances it is very desirable, as an assistance to the eye in its judgment of the absolute 
identity or otherwise of the position of lines, that the bright lines of comparison should 
not merely meet the dark lines in the star-spectrum, but that they should overlap them 
to a small extent. When the two spectra are so arranged as to be in contact, the eye is 
found to be influenced to some extent by the apparent straightness or otherwise of the 
compound line formed by the coincident, or nearly coincident lines in the two spectra. 
Owing to the unavoidable shortness of the collimator the lines in a broad spectrum are 
slightly curved. From this cause the determination of the identity of lines in spectra 
which are in contact merely is rendered more difficult, and it may be less trustworthy. 
The difficulties of observation which have been referred to were in the first instance 
sought to be overcome by placing the spark before the object-glass of the telescope. In 
some respects this method appears to be unexceptionable, but there are disadvantages 
connected with it. The bright lines, under these circumstances, extend across the star- 
spectrum, and make the simultaneous observation of dark lines, which are coincident, 
or nearly so with them, very difficult. When the spark is taken between open electrodes, 
the consequent disturbance of the air in front of the object-glass is unfavourable to good 
definition. An important disadvantage arises from the great diminution in the bright- 
ness of the spark from the distance (10 feet) at which it is placed from the slit ; since in 
consequence of its nearness to the object-glass, the divergence of the light from it is 
diminished in a small degree only by that lens. It is obvious that, by means of a lens of 
short focal length placed between the spark and the object-glass, the light from the spark 
might be rendered parallel or even convergent ; but the adjustments of such a lens, so 
that the pencils transmitted by it should coincide accurately in direction with the optical 
axis of the telescope, would be very troublesome. When two Leyden jars, connected as 
one jar, were interposed, and the spark was taken in air between platinum points, there 
was visible in the spectroscope only the brightest of the lines of the air-spectrum, namely, 
the double line belonging to nitrogen, which corresponds to the principal line in the 
spectra of the gaseous nebulae. When a vacuum-tube containing hydrogen at a low 
tension was placed before the object-glass, the line corresponding to F was seen with 
sufficient distinctness, but the line in the red was visible with difficulty. Some obser- 
vations, however, have been made with the spark arranged before the object-glass. 
The following arrangement for admitting the light from the spark appeared to me to 
be free from the objections which have been referred to, and to be in all respects adapted 
