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influence of coloured gaseous bodies. Iodine vapour was one of 

 these, and its action was found of a similar character to that of fluids 

 having a similar tint. Nitrous acid gas presented a far more extra- 

 ordinary phenomenon. 



The Spectrum of Newton, and of all the philosophers of the 18th 

 century, was a parallelogram of light with circular ends, in which 

 the seven colours gradually shaded into each other without any inter- 

 ruption. The illumination was a maximum in the yellow rays, and 

 the light decayed by insensible degrees towards the red and violet 

 extremities. In the year 1808, Dr Wollaston conceived the happy 

 dea of examining a beam of day-light that passed through an aper- 

 ture only the 20th of an inch wide, and he was surprised to see it 

 crossed by seven dark lines, perpendicular to its length. 



About ten or twelve years afterwards, the celebrated optician, 

 Joseph Fraunhofer, without knowing what had been done by Dr 

 Wollaston, observed the spectrum formed by the sun's light trans- 

 mitted through small apertures ; and by applying a telescope be- 

 hind the prism, he discovered about 600 parallel dark lines travers- 

 ing the spectrum. As no such lines appeared in the spectra of 

 white flames, Fraunhofer considered them as having their origin in 

 the nature of the light of the sun. The strongest of these lines were 

 seen in the spectra of the Moon, Mars, and Venus ; and by means 

 of very fine instruments, he was able to detect one or two of thorn 

 with other new lines in the spectra of Sirus and Castor. 



Upon examining with a fine prism of rock-salt, with the largest 

 possible refracting angle (nearly 78^), the light of a lamp transmit- 

 ted through a small thickness of nitrous acid gas, whose colour was a 

 pale straw-yellow, the author was surprised to observe the spectrum 

 crossed with hundreds of lines or bands far more distinct than those 

 of the solar spectrum. The lines were sharpest and darkest in the 

 violet and blue spaces, fainter in the green, and extremely faint in 

 the yellow and red spaces. Upon increasing, however, the thickness 

 of the gas, the lines grew more and more distinct in the yellow and 

 red spaces, and became broader in the blue and violet, a general ab- 

 sorption advancing from the violet extremity, while a specific absorp- 

 tion was advancing on each side of the fixed lines in the spectrum. 

 It was not easy to obtain a sufficient thickness of gas to develop the 

 lines at the red extremity, but the author found that heat produced 

 the same absorptive power as increase of thickness, and by bringing 

 a tube containing a thickness of half an inch of gas to a high tempe- 

 rature, he was able to render every line and band in the red rays 

 distinctly visible. 



