548 Intelligence and Miscellaneous Articles. 



grating. . All the observations were made in this speclruin on 

 account of its superior definition. 



An image of the sun about 1 centim. in diameter was formed on 

 the slit of the instrument by aid of a large Foueault heliostat, and 

 an achromatic lens of about four inches aperture. Appliances 

 were provided for moving the image laterally across the slit, and, 

 by means of a total reflecting prism, for turning it about the direc- 

 tion of the beam as an axis to bring any desired point of the limb 

 over the slit. 



The D 3 line appeared only when the suns image w T as tangent 

 to the slit, and then as a bright but very short line in the centre 

 of the field of view vertically considered. Its definition and inten- 

 sity were fouud to vary greatly from day to day, and for different 

 points on the sun's limb. In general, w r hen a solar prominence 

 lay across the slit the line was very broad and intense; but the 

 definition of its edges was poor, thus rendering it impossible to 

 set the cross-hairs on it with accuracy. The best combination of 

 intensity and definition was obtained by avoiding prominences and 

 working only on very clear days. 



The observations were made by the ordinary inicrometric method, 

 the D 3 line beiug compared with the best solar standard lines in 

 the field of view. The wave-lengths of these standard lines, as 

 taken from Prof. Rowland's " Table of Standard Wave-lengths "f , 

 were : — 



Fe 5862-580 



Fe 5859-810 



Ba 5853-903 



Average value. 5887' 028 



Fe 5916-475 



Fe 5914-384 



Fe 5905-895 



Na.D x 5896-164 



Seventeen series of measurements were made, in each of w^hich 

 equal numbers of observations were taken on diametrically oppo- 

 site points of the sun's limb in order to eliminate the effect of 

 rotation. 



The wave-length of D 3 was calculated from each of these series 

 by Prof. Rowland's method of interpolation, on the assumption 

 that, for the space used, the spectrum was essentially normal. 

 The average of the seventeen values thus found gives 



5875-939 ±-006 



for the wave-length of the D 3 line, the probable error being calcu- 

 lated from the deviations of the several values from the mean in 

 the usual manner. 



To test the accuracy of the observations and method of calcula- 

 tion, the wave-length of the mean liue was computed from the 

 observations and found to be 5887*027, a value which differs only 

 by -001 from the average of the wave-lengths of the standard 

 lines used, 5887*028. 



I am indebted to Prof. H. A. Rowland and Dr. J. S. Ames for 

 ) permission to use apparatus and for suggestions, and to Mr. W. S. 



I Day for aid in making the observations. — American Journal of 



i Science, November 1895. 



* Astr. and Astro.-Phys. vol. xii. p. 321 j and Phil. Mag. [5] xxxvi. 

 p. 49. 



