538 Scientific Proceedings, Royal Dublin Society. 



to the difference of their wave-lengths, and by measuring the 

 former the latter can be ascertained. Thus, an interval of one 

 degree on Angstrom's or Rowland's map indicates a difference of 

 wave-length amounting to a tenthet-metre, which, as we know, is 

 the ordinate of our standard gauge at a distance of a millimetre 

 from its end. But lines have been seen to be double with Prof. 

 Rowland's gratings, in which the separation of the two constituents 

 is not more than from 1/30*^ to 1/100*^ part of a degree. In the latter 

 case the difference of wave-length is only one twelfthet-metre. 

 This is the ordinate of our gauge at a distance from its apex 

 which is little more than the diameter of a single blood corpuscle, 

 and may be taken to be the smallest measurement that can as yet 

 be directly effected with certainty. 



The following is a list of close double lines which I have 

 myself seen in the solar spectrum with a small Rowland's grating ^ : 



The solar line 63, and a multitude of other close doubles, 

 shown as such on Rowland's map (2nd Series, 1888). 



The less refrangible of the two E lines. 



The line in the E group at X = 5264*4 of Rowland's scale. 



The nickel line which is nearly midway between the two 

 D lines. 



The least refrangible constituent of the triple lines at X = 

 5328-7. 



This last is about the closest double that my spectroscope will 

 resolve. There is no micrometer on my instrument, so that I 

 cannot give measures, but I estimate the coarsest of these, those 

 first mentioned, to have a difference of wave-length under two 

 eleventhet-metres — and in the closest, that last mentioned, it cannot 

 be more than a very few twelfthet-metres. Most of them could 

 be measured with a good micrometer. 



This can be accomplished with one of the smaller of Professor 

 Rowland's splendid gratings ; and he himself and other observers 

 have carried matters further, by taking photographs with the best 

 of his great six-inch concave gratings. This may give some idea 



1 The grating is flat, nearly an incli and three-quarters long, contains about 25,000 

 lines, and the observations were made in the fifth spectrum. In this spectrum the 

 image is formed by bringing together every fifth wave of light out of a series of 

 125,000 consecutive waves, and the ruKng must be sufficiently accurate to effect this. 



