428 



Mr. J. N. Lookyer. On some Phenomena [Mar. 20, 



Photographs of the spectra of Sr, Cu, and Mg, showing central thickening. 



I. Sr, containing two lines, one Ba showing very little thickening, while 

 the other true Sr line exists only at the centre as ahroad, fiuffly reversed 

 line. 



II. Cu, all the Cu lines in this photograph have their central portions ex- 

 panded, and most of them to a greater extent on the less than on the 

 more refrangible side. Some of the Fe lines have also their centres ex- 

 panded, which is not the case with the Ca lines, the Ca lines in the blue 

 being developed at their extremities. 

 III. Mg, showing line in the blue-green, with a central expansion on the less 

 refrangible side, giving the line the appearance of a half spindle, b 

 being quite normal. 



Spectrum of Sn ; showing lines of Ca and Fe ; the former are expanded at their 

 lower extremities, the Fe lines are not so expanded, but exist at a higher 

 level than the Ca lines. The photograph shows that the line whicb is 

 basic to Fe and Ca is carried up with the other Fe lines, and is also 

 expanded at its lower extremity with the other Ca lines. 



II. " Note on some Phenomena attending the Reversal of 

 Lines." By J. Norman LoCKYER, F.R.S. Received March 

 5, 1879. 



In the " Phil. Trans." for 1873, page 253, I gave an account of an 

 experiment devised hy Dr. Frankland and myself, in which the ab- 

 sorption line of sodium was made to vary considerably in thickness, 

 owing to the variation in the quantity of sodium vapour which was 

 produced in a tube when a mass of metallic sodium was heated in an 

 atmosphere of hydrogen. 



In the " Phil. Trans." for 1874, vol. clxiv, Part II, p. 805, speak- 

 ing of the photographs of arc spectra which I had then commenced 

 to take, I stated, " it not unfrequently happens that a very thick line 

 will reverse itself, a circumstance which greatly facilitates its com- 

 parison with confronted lines, since a thin dark line then runs down 

 the centre of the thicker bright one," and I pointed out in a note that 

 the absorption line does not always occupy the exact 'centre of the 

 bright band. I gave examples of this from the spectra of calcium 

 and aluminium, the examples beiug reproductions of photographs of 

 the arc. These were published with the paper. 



In other subsequent communications, I have referred to these 

 reversals, and I have elsewhere made general statements, with regard 

 to them, and drawn attention to the distinction between those sub- 

 stances which give us winged lines in arc spectra and those which do 

 not. 



If the method of throwing an image of the arc upon the slit be 

 employed, a method which I suggested and utilised in 1870,* for 

 * " Phil. Trans.," 1873, p. 254. 



