Table of Standard Wave-Lengths. 479 



The only disturbance now found was due to heat from 

 the wire below the glass box containing the needle, and to 

 get over this Davies rigged up a number of concentric cylin- 

 drical jackets of tin plate round the box containing the needle. 

 These certainly constituted an electrostatic screen. Never- 

 theless, I had hope that though such a screen might diminish 

 the effect it would not destroy it. 



I had a letter from Davies yesterday saying that a very 

 minute effect could be seen with the screens all on, and that 

 it did everything properly as regards reversals. On taking off 

 one screen it increased. On taking off another it increased 

 more, but showed signs of superposed spuriousness. On 

 taking oft' the third screen the effect was masked by spurious 

 actions. 



Everything points to the fact, therefore, that we have now 

 several times observed the true effect, and I entertain practi- 

 cally no doubt of it. 



But inasmuch as some of the observations have been made 

 so lately, and some even since I left Liverpool this time, I 

 would not wish the Society to suppose that I regard the 

 research as finished and complete. I should like to clear up 

 distinctly the effect of screens. 



I must speak in high praise of the skill and neat-fingeredness 

 of my laboratory assistant Mr. Davies in carrying out the 

 rather troublesome requirements which this finnicking re- 

 search has at various times seemed to necessitate. 



LY. Table of Standard Wave-Lengths. 

 By Professor H. A. Eowland *. 



IN the ' American Journal of Science ' for March, 1887, and 

 the Phil. Mag. for the same date, I have pubHshed a 

 preliminary Hst of standards as far as could be observed with 

 the eye, with a few imperfectly observed by photography, the 

 whole being reduced to Bell's and Peirce's values for absolute 

 wave-lengths. Mr. Bell has continued his measurements and 

 found a slightly greater value for the absolute wave-length of 

 the D line, and I have reduced my standards to the new 

 values. 



Nearly the whole list has been gone over again, especially 

 at the ends around the A line and in the ultra-violet. The 

 wave-lengths of the ultra-violet were obtained by photographing 



* From the ' Johns Hopkins University Circular ' for May, 1889. 

 Communicated by the Author. 



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