On a Sensitive-strip Spectropolariscope. 161 



considering, which is rapidly damped, the time for a half- 

 oscillation would increase as the discharge died away, a result 

 experimentally obtained and published in a letter written to 

 ' Nature 3 by the author in August 1900. 



An interesting application of the method is the determination 

 of the deflexion that would be attained by a ballistic galvano- 

 meter when a discharge passes through it which cannot be 

 assumed to have ceased flowing before the needle has moved. 



In this case the value of — would be variable, and the sub- 

 traction would have to be repeatedly performed from a 

 variable point. 



In conclusion I wish to express my indebtedness to Mr. Gr. 

 W. Worrall, B.Sc, for working out some of the values of 

 the self-induction under varying currents which have been 

 used in plotting the curves given above. 



University College, Liverpool. 



XI. A Sensitice-strip Spectropolariscope .] 

 By Professor D. B. Brace *. 



THE conditions for maximum sensibility of the eye in 

 making comparisons with the polariscope are the same 

 as those of a photometer. The entire displacement, except in 

 technical work, of photometric comparisons with ordinary 

 light by spectral photometric observations, which the growing 

 demands in photometry for exact data on specific colours 

 have produced, illustrates the corresponding condition in 

 polariscopic work. The importance, both theoretical and 

 practical, of determining, on the one hand, the relative 

 distribution of intensities subjectively and objectively in any 

 radiant and, on the other, the relative transmission of any 

 absorbent for different periodicities, has necessitated the 

 highest instrumental refinements. 



It is now possible to obtain settings for the mean spectral 

 colours with a probable error of less than one-fifth per cent, f 

 and on some occasions with a carefully trained eye as low as 

 one-tenth per cent. This has been brought about by 

 improving the viewing screen so that the bounding lines 

 between the comparison fields should be perfectly sharp and 

 vanish with equal illumination of the fields, the colour of 

 course becoming the same over the entire field of view. In 



* Communicated by the Author. Read before the American Physical 

 Society at its Pittsburg meeting', July 1902. 

 t Phil. Mag-. [5] xlviii. p. 420 (1899). 



Phil. Mag. S. 6. Vol. 5. No. 25. Jan. 1903. M 



