332 Prof. Piazzi Smyth on the Constitution of the Lines 



one eighth and one ninth of the same space; while the salt- 

 line double itself is only one eighth of the average distance 

 apart of the stronger flutings of the citron-band of the carbo- 

 hydrogen blowpipe-flame, which band has some six or seven 

 of such flutings within its easily perceivable breadth. 



Or, again, if we should on the black board represent the 

 separation of the first and second of any oxygen triplet by a 

 tenth of an inch, and from the second 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 Q is rather more than three times as long 

 as the late Professor Angstrom's grand and almost universally 

 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 

 repetitions, 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 failed 

 in several ways. So lately I entered into a contract with 

 an exceedingly skilful as well as persevering optician in 

 London, viz. Air. Adam Hilger, to make two large bisulphide 

 prisms, having a clear circular aperture of 2*1 inches 

 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. HiJger had to go through were 

 almost overwhelming. He bored through block after block 

 of crown glass, only to find after an hour, or a day, or a week 



