466 



MR. A. FOWLER ON THE SPECTRUM OF MAGNESIUM HYDRIDE. 



been employed in the case of the laboratory spectrum, lines have been considered 

 coincident when they agreed within 0'033 of a tenth-metre. It will suffice to 

 summarise the results of this comparison, which covers the region 5180'7 to 5090'2. 



In this region there are 55 lines of intensity 5 and upwards in the magnesium 

 hydride spectrum; 31 of these, or 56 per cent., agree within the specified limit with 

 solar lines; 19, or about 35 per cent., do not agree within this limit; and 5 are 

 certainly or possibly masked by metallic lines. 



In the same region there are also 55 magnesiiim hydride lines of intensity and 

 less, solar coincidences with which must be regarded as accidental. Of these, 22, or 

 40 per cent., agree with solar lines within the limit stated; 31, or 56 per cent., do 

 not agree ; and 2 are probably confused with metallic lines. 



The difference in the representation of the two groups of lines is perhaps sufficient 

 to indicate that the stronger lines of magnesium hydride may be really present as 

 very faint lines in the Fraunhofer spectrum, but the evidence is less convincing than 

 might be desired, especially as some of the strongest magnesium hydride lines do not 

 appear in ROWLAND'S catalogue. There is, however, some doubt as to the completeness 

 of ROWLAND'S record of the very faint lines,* and until this is removed no final 

 conclusion can be reached. 



Another attempt to obtain evidence upon this point has been based upon the series 

 lines to which reference has already been made. ROWLAND'S relative wave-lengths 

 might be expected to be sufficiently accurate to show nearly constant second differences 

 for lines corresponding to a given series, if the supposed solar coincidences were really 

 significant, but this test cannot be very effectively applied in consequence of the 

 frequent interruption of the possible series by superposed metallic lines. In the case 

 of the series, however, there are five consecutive lines which are free from this 

 complication, details of which are given in the appended table : 



JEWELL, 'Trans. Internl. Union Sol. Res.,' vol. 1, pp. 49, 50. 



