676 DR. W. HUGGINS ON THE PHOTOGRAPHIC SPECTRA OF STARS. 
The photographic spectra of the brighter stars can be traced upon the plate from 
about b to beyond S, but in the accompanying map I have limited myself to the 
portion of the spectrum between the line of hydrogen (y) near G and 0 in the ultra¬ 
violet. 
An admirable wire micrometer by Dollond, attached to a microscope furnished 
with a two-inch objective, was used to measure the photographs. The readings of 
the micrometer head give 2'947 liundreths of a revolution for each '0000001 m.m. of 
wave length at the position of H. 
By means of photographs of the solar spectrum, and of those of the spectra of iron, 
cadmium, calcium, and magnesium, a curve on a sufficiently large scale was laid down 
on paper ruled in millimetres connecting the measures of the micrometer with the 
intervals of wave length. Great care was taken by cross measurements in different 
ways to make this curve as accurate as possible. The positions of the lines as 
determined in wave lengths were afterwards confronted with solar lines by actual 
measurement under the microscope. I do not think the probable error of the deter¬ 
mination in wave lengths exceeds in any case dr 2 ten millionths of a millimetre. For 
most of the lines I think it is less than half of this amount. 
§ VII. Results. 
It need hardly be mentioned that only nights of great atmospheric clearness are 
suitable for stellar photography. The unusual prevalence of unfavourable weather 
during the time this work has been in hand has greatly limited tire number of success¬ 
ful photographs I have been able to obtain. The remarkable circumstance of the 
apparent absence of the line K in one of my earlier photographs of Sirius, made me 
select, in the first instance, other stars belonging to the same class. 
In the accompanying map I have given of this class of stars the spectra of Sirius, 
Yega (a Lyrse), a Cygni, a Virginis, rj Ursse Majoris, and a Aquilae, and representing 
a different class of stars the spectrum of Arcturus. In addition to these stars I have 
obtained photographs of /3 Pegasi, Betelgeux, Capella, a Herculis, and a Pegasi; but 
as these are more or less incomplete, in consecpience of the unfavourable state of the 
atmosphere when they were taken, I prefer to reserve any description of their spectra 
for the present. 
I have obtained good spectra of the planets Venus and Jupiter, taken together with 
a broader spectrum of daylight for comparison, and also of Mars. 
Numerous photographs of limited areas of the moon’s surface have been taken 
under different conditions of illumination, and also of the moon during a partial 
eclipse of that body. 
Besides the above objects there are several directions in which celestial spectrum 
photography could doubtless be applied with great advantage. One of these, which 
the bad weather alone has prevented me from attempting, was to supplement my 
former eye observations of the spectra of gaseous nebulae by photography. As the 
