208 
POPULAR SCIENCE REVIEW. 
exists in the Yellow ray, and it diminishes as we recede 
from it towards either end of the spectrum. The least refran- 
gible, or the Red rays, give a modified amount of Light ; but the 
maximum of Heat exists in them. The most refrangible, or 
Blue end of the spectrum, is less luminous ; but the maximum 
of chemical action is fixed at this extremity, the fluorescent 
rays beyond the spectrum of Newton being visible only under 
the peculiar circumstances mentioned. 
When these beautifully coloured bands of Light are well defined 
upon a screen, let the inquiring reader examine the image with a 
small telescope. A new set of phenomena will become appa- 
rent. The spectrum is then seen to be crossed by a vast 
number of black lines. Every ray, even the most brilliant, will 
be seen to have spaces in which there is an entire absence of 
light. To these we would now direct attention. 
It is instructive to trace the steps by which we slowly 
advance to the discovery of a great truth. As in ascending a 
tall column, the way may, for a season, prove dark and possibly 
- — being without promise — wearying; but, eventually, the 
gleams of light are seen, and presently a wide horizon is 
opened to our view. Thus has it been with this inquiry. 
Dr. Wollaston was the first who observed the existence of 
non-luminous spaces in the prismatic spectrum. Dr. Ritchie 
proved that these lines were dependent on absorption, and 
showed how they could be increased in visible numbers by 
artificial means. Fraunhofer, however, was the first to make a 
full investigation of these lines, and to publish a map of them ; 
hence they have been generally called Fraunhofer’s lines. 
These lines are of so fixed a character in relation to the 
coloured bands of the spectrum, that if it is desired to indicate 
with great precision any special ray of the spectrum, we refer to 
them by their letters or numbers. In the accompanying plate, the 
more remarkable lines only are given. The positions they occupy 
have been determined, by a careful examination of the map of 
Fraunhofer, and the very complete delineation of those lines 
published in the “ Philosophical Transactions for 1859,” by Sir 
David Brewster and Dr. Gladstone. Fraunhofer laid down on 
his map 354 lines, but Sir David Brewster says — “ In the 
delineations which I have executed, the spectrum is divided 
into more than 2,000 visible and easily recognized portions, 
separated from each other by lines more or less marked.” 
The lines marked with capitals from A to I in the plate may 
be always easily detected in any Solar spectrum. Those which 
are indicated by small letters, and those which are numbered, 
are the more marked lines observable in Sir D. Brewster’s map ; 
the letters and numbers agreeing with those which he employs. 
The origin of the dark lines — spaces in which there is no 
Light — can scarcely be said to be yet resolved. Fraunhofer, and 
