1 82 Prof. W. Gibbs on the Measurement of Wave-lengths, 



being taken from Ditscheiner's* measurements of the indices of 

 a flint prism by Steinheil, of refracting angle 60° 4' 59" : — 



Kirchhoff's x Angular ^ T ,. ^ 



, A. i °,- A. Indices. a. 



scale. deviations. 



B . 593 687-06 47 40 55 ... 1-61358 



C . 694 655-95 47 51 19 655-97 1*61537 655*8.2 

 . 877 613-57 48 8 8 ... 1-61824 



From this it appears that the error in the determination of 

 the wave-length of the middle line C is only -f0'02 when the 

 angular deviations are employed, but amounts to —0*13 when 

 the indices of refraction are taken as the elements of the calcu- 

 lation. Yet the interval between B and 877 is very large. 



The following data are taken from another part of the scale, 

 the measurements being made with the same prism :-— 



Kirchhoff's x Angular x Indices ^ 



scale. deviations. 



b . 1648-8 517-13 49 4 16 ... 1*62775 



16556 516-58 49 4 44 51656 1*62782 516-61 

 1693-8 514-08 49 6 47 ... 1-62817 



Hence the error in the determination of the wave-length of 

 1655*6 is, when the angular deviations are taken, only —002, 

 and when the indices are taken -f 003. It must be borne in 

 mind that in all the above-mentioned examples the angles are 

 those of minimum deviation. As the numbers upon Kirchhoff's 

 scale also represent angular (though not minimum) deviations, it 

 seemed worth while to determine how far for a short interval 

 these could be employed. Taking the three scale-numbers of 

 the last example, the error in the wave-length of 1655*6 was 

 found to be —0*38; and when the scale-numbers were taken as 

 the sines or tangents of corresponding angles, -f- 0*09. 



The following data are taken from the more refrangible part 

 of the spectrum, the measurements being also those of J)it- 

 scheiner, and made with the same prism :— 



2822-8 433-34 50 34 57 ... 1-6-1287 

 G . 2854-7 430*88 50 37 52 43068 164334 430*83 

 2969-7 429-90 50 38 47 ... 1-61352 



In this case the error in the wave-length of the middle line 

 "(2854*7) is —0*20, as determined from the angular deviations, 

 and —0*05 as determined from the indices. It must be borne 

 in mind that, in this part of the spectrum, the determination 



* Bestimmung der Wellenlangen der Fraunhoferschen Linien des Sonnen* 

 spectrums, p. 43. t 



