CELESTIAL SPECTROSCOPY. 5 
absorption spectrum of oxygen—the bands may have an intensity pro- 
portional to the square of the density of the gas, and may be due either 
to the formation of more complex molecules of the gas with increase of 
pressure or, it may be, to the constraint to which the molecules are sub- 
ject during their encounter with one another. 
It may be thought that atleast in the coincidences of bright lines we 
are on the selid ground of certainty, since the length of the waves set 
up in the «ther by a molecule, say of hydrogen, is the most fixed and 
absolutely permanent quantity in nature, and is so of physical necessity, 
for with any alteration the molecule would cease to be hydrogen. 
Such would be the case if the coincidence were certain; but an abso- 
lute coincidence can be only a matter of greater or less probability, 
depending on the resolving power employed, on the number of the lines 
which correspond, and on their characters. When the coincidences are 
very numerous, as in the case of iron and the solar spectrum, or the 
lines are characteristically grouped, as in the case of hydrogen and the 
solar spectrum, we may regard the coincidence as certain; but the 
progress of science has been greatly retarded by resting important con- 
clusions upon the apparent coincidence of single lines in spectroscopes 
of very small resolving power. In such cases, unless other reasons 
supporting the coincidence are present, the probability of areal coinci- 
dence is almost too small to be of any importance, especially in the case 
of a heavenly body which may havea motion of approach or of recession 
of unknown amount. 
But even here we are met by the confusion introduced by multiple 
spectra, corresponding to different molecular groupings of the same 
substance and, further, to the influence of substances in vapor upon 
each other; for when several gases are present together the phenomena 
of radiation and reversal by absorption are by no means the same as 
if the gases were free from each other’s influence, and especially is this 
the case when they are illuminated by an electric discharge. 
I have said as much as time will permit and I think indeed suffi- 
cient to show that it is only by the laborious and slow process of most 
cautious observation that the foundations of the science of celestial 
physics can be surely laid. We are at present in a time of transition, 
when the earlier and, in the nature of things, less precise observations 
are giving place to work of an order of accuracy much greater than 
yas formerly considered attainable with objects of such small bright- 
ness as the stars. 
The aceuracy of the earlier determinations of the spectra of the ter- 
restrial elements is in most cases insufficient for modern work on the 
stars as well as on the sun. It falls much below the scale adopted in 
Rowland’s map of the sun, as wellas below the degree of accuracy at- 
tained at Potsdam by photography in a part of the spectrum for the 
brighter stars. Increase of resolving power very frequently breaks up 
into groups, in the spectra of the sun and stars, the lines which had 
