and on those produced by the Earth's Atmosphere, §c. 3857 
tubes in cylinders of tinned iron with narrow slits to admit 
the light, there is little danger of any serious accident. 
When the gas is in the liquid state, it produces none of the 
fixed lines which I have described, and exercises no other 
action upon the spectrum than any ordinary fluid of the same 
orange colour. 
In examining the structure of the solar spectrum, Fraunho- 
fer seems to have put forth all his strength in determining 
the position of the principal lines, A, B, C, D, E, 6, I’, G and 
H*, which he had selected as equidistant as possible, for the 
purpose of measuring their angular distances in different 
media, and thus obtaining the most accurate data for the con- 
struction of the achromatic telescope. ‘These measures he 
has given with the greatest exactness for various kinds of 
crown and flint glass and for a few fluids, and be has thus put 
it into the power of the practical optician to construct achro- 
matic object-glasses, with a degree of certainty and perfection 
hitherto unknown. 
This method, however, notwithstanding its high value, is 
not easily applicable in practice, and from the nice observa- 
tions which it involves, we have reason to believe that it has 
not been used by any other artist than Fraunhofer himself. 
The difficulty of procuring out of the mass of glass to be em- 
ployed, prisms sufficiently pure to show such narrow lines as 
E, or the two which constitute D+,—of obtaining the sun 
when his light is wanted, and of observing and measuring the 
distances of the fixed lines ina spectrum constantly in motion, 
are insurmountable obstacles to the general adoption of so 
refined a method of measuring dispersive powers. 
From all these difficulties, the discovery of lines in the ni- 
trous acid gas spectrum completely relieves us. As the lines 
whose distances are required, may be made as broad and 
black as we please, prisms of ordinary purity are suflicient to 
exhibit them in perfect distinctness. The artificial light of a 
lamp can be commanded at any hour, and as its rays are ab- 
solutely fixed, the least experienced observer can have no dif- 
ficulty in measuring the distances of the fixed lines, and thus 
obtaining, with extreme accuracy, all the data for the con- 
struction of achromatic instruments. 
But it is not merely to this practical purpose that the 
gaseous lines are singularly applicable. Among the various 
solids and fluids in nature, there are very few sufficiently pure 
and transparent, to enable us to see through them the lines of 
* Six of these, viz. B, D, 6, F, G, and H, were discovered by Dr. Wollaston. 
+ These lines are also the most important, as the most luminous part of 
the spectrum lies between them. 
2R-2 
