302 F. J. ltogers — Magnesium as a Source of Light. 



spectra being viewed in the usual manner by means of an 

 observing telescope. The advantage of this form of spectro- 

 photometer is that there is no polarizing device, equality of 

 the particular portions of the spectra compared being obtained 

 by movement along the photometer-bar. . 



The light to be studied was produced by the combustion of 

 magnesium-ribbon 2A- mm wide, supplied at a uniform rate by 

 means of clock-work. The light was very variable, a fact 

 which greatly increased the difficulty of determining its qual- 

 ity. Even when a diaphragm with an opening of 2x5 mm 

 was placed immediately in front of the burning ribbon this 

 variation in intensity was quite marked. The best that could 

 be done was to take the mean of a large number of observa- 

 tions. The comparison was made at eight points very nearly 

 equidistant in the normal spectrum. Five sets of observations 

 were made making in all fourteen readings for each of the 

 eight points in the spectrum. Table I gives the ratios, mag- 

 nesium light : gaslight, reduced to unity at the D line, as 

 obtained by taking the mean of these five sets of observations. 







Table I. 







"Wave- 



Mg. light. 





Wave- 



Mg. light. 



engths. 



gaslight. 





lengths. 



gaslight. 



•450 



8-77 





•574 



1-21 



•479 



5-33 





•606 



0-83 



.506 



3-43 





•635 



0-66 



•536 



2-07 





•670 



0-53 



The full curve in fig. 1 represents graphically the relation 

 between wave-lengths and relative intensity in the case of 

 the magnesium light. The standard gaslight is arbitrarily 

 taken to be unity in all parts of the spectrum. Curve 

 II*, fig. 1, represents the relative intensity of the arc 

 light for different wave-lengths as obtained by Dr. Nichols 

 in 1888. Curves III and IY, fig. 1, represent the relative 

 intensity of daylight as compared with the standard. Ill 

 represents daylight from a cloudless sky- and IY, daylight from 

 a heavily clouded sky. In the latter case the cloudiness hap- 

 pened to be-just sufficient to make daylight almost identical in 

 character with the magnesium light. For these dotted curves 

 the standard light was a sixteen candle power incandescent 

 lamp which, however was found to give a light nearly identical 

 in quality with the light from an Argancl gas burner. The 

 stars in the figure for X— -656, '527, and 450 , represent the 

 values for magnesium light obtained by Prof. Pickering in 



* This Journal, vol. xxxvii, p. 100. 



