1879.] oh served in the Arc produced by a Siemens' Machine. A27 



the central member alone is thickened along with the other calcium 

 lines on one of the poles. 



LIST OF PHOTOGRAPHS EXHIBITED. 



Photograph of Mn lengths taken in the old way, showing that with a battery of 

 30 Grove cells and a horizontal arc a perfectly symmetrical photograph 

 was obtained with the lines extending equally on either side of a hori- 

 zontal ink line drawn through the axis of the photograph. 



Photographs of Ba, Sr, and Mn, giving in each case the spectrum of the core compared 

 with that of the flame of the arc. 

 I. Ba, showing that of two lines (wave-length 4130'5 and 4282 - 5) of 

 equal intensity in the core, the less refrangible was visible almost alone 

 in the flame. 



II. Sr, showing two lines at 4078*5 and 4215'3. In the core the more 

 refrangible has a much broader reversal than the other, while in the 

 flame the less refrangible exists alone and reversed. 

 Ill and IV. Mn, in the flame-spectrum of one of these photographs the 

 triplet at about 4030"0 exists with no other Mn lines ; while it is 

 absent from the flame-spectrum of the other photograph, although the 

 Mn line at 4234'8 is present. 



Photograph of flame-spectrum of Ca taken with an oblique slit, as the flame nearly 

 always branches off at an angle. 



Spectrum of Kb, showing that the carbon bands cling to the hotter pole, while the 

 Ca and Rb lines cling to the opposite pole ; one set of carbon bands, 

 however, stretches right across the spectrum. 



Two photographs showing reversal of phenomena by reversing the current. 



I. Two spectra of Pb, obtained by normal current and reversed current, 

 showing that the lines which, in the upper spectrum were thickened at 

 their lower extremities, were in the lower spectrum thickened at their 

 upper extremities, and that the general appearance of the two sets of 

 lines was reversed. 

 II. Two spectra of Cu showing the same phenomena. 



Spectrum of Mn obtained with the large arc of a Siemens' machine, in which the 

 want of symmetiy is so conspicuous that if a straight line be drawn 

 through the centre of the photograph it will cut one set of lines at their 

 centres, another near their upper extremities, while a third set of lines 

 will be cut near their lower extremities. 



Photographs showing separation of lines. 



I. Spectrum of Li containing impurity lines of Ca, Sr, Ee, and Mn, the Ca 

 and Mn lines clinging to one pole, and the Ee and Sr lines to the other. 

 II and III. Different parts of Fe spectrum, showing Ca lines starting from 



one pole, and Fe lines from the other. 

 IV. Spectrum of Cu, showing that this separation can exist not only between 

 two metals, but even between lines of the same metal. In this the blue 

 Ca line is seen thickened at the upper pole, while the H and K lines 

 were only seen at the lower pole. 



Spectra of Ti, Ni, and Mn, in which the lines were not all produced at the two 

 poles, some occupying an intermediate position ; the lines thus arranging 

 themselves en echelon along the spectrum. 



Spectra of Cu and Ni, showing the irregular thickening of different lines at different 

 levels of the arc. 



2 i 2 



