On the Absolute Wave-length of Light. 265 



This table is to be used in connection with my photographic 

 map of the normal spectrum, to determine the error of the 

 latter at any point. The map was made by placing the pho- 

 tograph in contact with the scale, which was the same for 

 each order of spectrum, and enlarging the two together. In 

 this way the map has no local irregularities, although the 

 scale may be displaced slightly from its true position and may 

 be a little too long or short, although, so far as I have tested 

 it, it seems to have very little error of the latter sort. The 

 scale was meant in all cases, except the ultra-violet, to apply 

 to Peirce's absolute value, and so the correction is generally 

 negative, as follows : — 



Approximate Correction to the Photographic Map of the Normal 

 Spectrum to reduce to latest absolute value. 



Correction. 



Strip 3200 to 3330 -*05 



„ 3275 to 3530 -"05 



„ 3475 to 3730 -'02 



3675 to 3930 --10 



3875 to 4130 -'16 



4075 to 4330 -'04 



4275 to 4530 -'08 



4480 to 4735 --10 



>) 



4685 to 4940 -'18 



„ 4875 to 5130 --14 



„ 5075 to 5330 -*15 



„ 5215 to 5595 ... about -'05 



„ 5415 to 5790 ... about -'04 



„ 3710 to 3910 -'20 



„ 3810 to 4000 —-14 



It is to be noted that the third spectrum of the map runs 

 into the second , so that it must not be used beyond wave- 

 length 3200, as it is mixed with the second in that region. 



XXX. On the Absolute Wave-length of Liglit. By Louis 

 Bell, Fellow in Physics in Johns Hopkins University .* 



o 



UP to the present time, Angstrom's map of the solar 

 spectrum, and with it his determination of absolute 

 wave-length, has remained the final standard of reference in 



o 



all spectroscopic matters. But since Angstrom's work was 

 published, optical science, particularly that part of it which 

 deals with the manufacture and use of diffraction-gratings, 

 * Communicated by the Author. 



