FORMING THE LOW TEMPERATURE SPECTRUM OF OXYGEN. 421 
to the third by six-hundredths of an inch (when it will require a good eye from 
the other side of the table to separate them), that would indicate a scale for the 
whole spectrum, or from red to violet, of 25 feet; and that is rather more than 
three times as long as the late Professor ANGSTROM’s grand and almost univer- 
sally followed “ Normal Solar Spectrum.” 
For bright line, chemical, spectroscopy, and especially with the faint light of 
incandescent gas in a low temperature electric spark, it is by no means usual or 
easy to separate lines so very close together as the members of one of the 
oxygen triplets. A few words of explanation may, therefore, be demanded of 
me in proof that the resolution was real, and not an optical deception. The 
propriety of the demand, too, I am quite ready to allow, knowing only too well 
that there are prisms which will fringe every bright line with diffraction repeti- 
tions ; or, when out of the best focus, will double or treble any line, and others 
again that make them so broad and hazy that clear separation of very close 
lines would be utterly impossible. 
As my gas-vacuum tube spectroscope admits only of a deviation range up 
to 45°, or that of a single white-flint prism of 52° refracting angle, I was 
compelled to have recourse to compound prisms to get up the necessary dis- 
persion for crucial cases, say that of seven or eight such prisms. Now, although 
some beautiful compounds were made for me both in France and this country, 
they invariably failed in the item of perfect definition. I then tried a large fluid 
bisulphide-of-carbon prism made in Paris, but that failed in several ways. 
So lately I entered into a contract with an exceedingly skilful as well as 
persevering optician in London, viz., Mr. Anam Hitcer, to make two large 
bisulphide prisms, having a clear circular aperture of 2:1 inches in diameter, a 
refracting angle of 104°, anti-prisms of crown glass square at the ends, and 
a-central angular bored block of the same material to hold the fluid. 
The troubles poor Mr. Hincer had to go through were almost overwhelm- 
ing. He bored through block after block of crown glass only to find after an 
hour, or a day, or a week that it fell in pieces of its own accord, until he had a 
heap of tunnelled fragments large enough to make a cautionary photograph of. 
And when he had at last succeeded by his unconquerable perseverance m 
securing two blocks that stood, and the anti-prisms were to be fastened on 
their faces, he tried almost every patented and unpatented cement before he 
found one, or rather a particular method of using it, which could withstand the 
action of the bisulphide for more than a day or two. 
At last, however, though long after the contracted time, he brought the two 
prisms here complete, and they passed successfully through the severest trial I 
had prepared for them, viz., that when a hydrogen line was at its brightest, 
shining like a ray of sunlight in a dark field, its light must be entirely and 
