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to elucidate or confirm his observations — such were some of the 
qualities which were required in the physicist who was to place 
the science of spectral analysis (as applied to astronomy) upon a 
sound basis. When we compare the vague and unsatisfactory 
results obtained by other observers with those which Mr. 
Huggins has set before us, and when we note also the influence 
which his researches have had in showing the way to less expe- 
rienced observers, and enabling them to make important dis- 
coveries, we begin to understand the full value of his labours. 
We would say to those whom we are about to invite to a new 
and fertile field of labour. Let your first task be to examine the 
work which j\Ir. Huggins has done ; observe the skill with which 
the mode of observation is selected; note how each result is 
weighed and tested before it is admitted ; and, in fine, 
Vos exemplaria Grseca 
Nocturna versate manu versate diuma. 
The principles on which spectrum analysis depends are few 
and simple ; nor are the contrivances by which it is adapted to 
astronomical research difficult of comprehension. 
The simple prismatic analysis of light is exhibited in fig. 1. 
A beam of sun-light, ab,* passing into a darkened room through 
an aperture in a screen ss' would, if not interfered with, form 
a small spot of light at i, in the same straight line with ab. 
Now, suppose a prism p to be interposed. If sun-light were 
homogeneous, the laws of optics tell us that the beam would be 
simply diverted from its course, so that the spot of light would 
fall as at the dark line indicating the path of the ray. But, 
instead of this, a streak of light, as vr, is seen (vr is much 
exaggerated in length) coloured like the rainbow, red at its 
lower end (when the prism is situated as shown), and violet at 
its upper end — the colours succeeding each other in the order 
red, orange, yellow, green, blue, indigo, and violet. It is the 
analysis of this spectrum which forms the mode of research 
recently rendered available to the astronomer. The examina- 
tion of the dark lines which appear in the spectrum when the 
sun is the source of light is called solar spectroscopy, the 
determination of the position of the lines seen in the spectra of 
I have tlionght it best not to confuse tliis account by considering the 
size of the source of light, or by exhibiting the course of a slightly divergent 
pencil. Experience shows that the beginner is apt to be confused by such 
a mode of describing the formation of the spectrum. He confounds the 
dispersing effect of the prism with such effects as lenses produce on the 
convergency or divergency of pencils, because he sees the same mode of 
illustration used in each case. As the space at my disposal only permits a 
few words in reference to the formation of the spectrum, it was the more 
necessary that no mistake of this sort should arise. 
