TRANSACTIONS OF SECTION A. 583 



important for the photometry of coal-gas that the standard light employed should 

 be as nearly as possible of the same colour as the light of the gas-flame. The 

 light of the lamp has been brought to this colour by a small admission of air below 

 the point of combustion. Twelve holes, each .3 mm. in diameter, have been drilled 

 through the outer tube 15 mm. below the top. The draught of the chimney is 

 sufficient to determine an entry of air through these holes, which, happening under 

 tixed conditions, is constant in amount. A short cylindrical screen goes round the 

 chimney, the bottom of which is 58 mm. above the surface of the burner ; and both 

 screen and chimney are surrounded by a pentagonal shade, four panels of which 

 are filled with blue glass, while the fifth, which is turned towards the disc of the 

 photometer, is filled half-way down with a metal plate, which, overlapping the 

 inner screen, allows only the light from the lower part of the flame to fall on the 

 disc of the photometer. By supporting the lamp at such a height that the bottom 

 of the screen is level with the centre of the disc, errors of parallax are avoided. 

 Mr, Sugg, I believe, observed, and Mr. Dibdin has proved, that with the flame of 

 an argand burner the light thus given by the lower part of the flame is independent, 

 within wide limits, of the total height of the flame. The observation is true, not 

 only for a particular burner, but for many, and probably for most, argand 

 burners. Many alterations have been made in the structure of this lamp, aud it 

 has generally been found, though not always with equal exactness, that the height 

 of the flame did not affect the light emitted from its lower part. As the lamp is 

 now arranged the height of the flame can be observed through the blue glass 

 panels, and whether the top is just visible above the circular screen or rises to the 

 top of the panel, the light which falls upon the disc of the photometer does not 

 alter measurably. The adjustment of the light to the value of ten candles — in 

 other words, the adjustment of the height of the circular screen — has been efl'ected 

 by a number of comparisons between the lamp and the one-candle pentane 

 standard. 



1 i. On the Cause of the Spurious Double Lines sometimes seen with Spectro- 

 scopes, and of the Slender Ajjpendages tvhich accompany them. By 

 G. Johnstone Stoney, M.A., D.Sc, F.R.S. 



Spurious double lines are sometimes seen with the spectroscope. In order to 

 observe the phenomenon it is not necessary to use the complete spectroscope, noi 

 is monochromatic light essential. It is sufficient to look with the telescope of 

 the instrument directly down the collimator, so as to see the image of the slit. 

 If the instrument be now pointed towards, or nearly towards, a distant flame, 

 aud if the slit be narrowed down to a certain point, a spurious double line will be 

 seen in the observing telescope, instead of a correct image of the slit. The 

 phenomenon will be produced under the circumstances which most readily admit 

 of investigation when the incident light is restricted to a single beam of plane 

 waves, falling on the slit either normally or obliquely ; and incident light which 

 sufficiently approximates to being of this kind is easily provided by placing a 

 coarse supplementary slit in front of the lamp flame and allowing only the light 

 which passes through this slit to reach the collimator. 



Under these circumstances it will be found that when the light falls normally 

 on the slit of the spectroscope it will form an image in the observing telescope 

 which, with a certain width of slit, becomes a rather coarse double line bordered 

 on either side by an exceedingly fine hair-like appendage, which is visible only 

 when the light is sufficiently intense. By causing the incident light to fall 

 obliquely the two constituents of the double line may be made to thin down, 

 leaving a considerable dark interval between them, and then present verv much 

 the appearance of the sodium lines when fine. In intermediate positions of the 

 incident light the interval between the two constituents of the double line is 

 occupied by a bright ruling of hair-like appendages, varying in number with the 

 inclination of the incident light. The conditions of the experiment may be 

 modified in other ways, and other appearances produced — notably a flare con- 

 sisting of a ruling of bright lines fading out in one direction. 



