TABLE OF STANDARD WAVE-LENGTHS OF THE SPECTRAL LINES 551 



hundred standard lines in the visible spectrum, including these primary 

 standards, with a micrometer having a range of five inches, and very 

 accurately made. The spectrum being strictly normal, the readings so 

 made were proportional to the wave-length. They could have been used 

 simply to interpolate between the primary standards, but I preferred 

 another method. The readings of the micrometer were made to over- 

 lap, so that, by adding a constant to each set,- a continuous series could 

 be formed for the whole spectrum which would be proportional to the 

 wave-length except for some slight errors due to the working of the 

 apparatus for keeping the focus constant. Making this series coincide 

 with two standards at the ends, the wave-lengths of all could be obtained 

 by simply multiplying the whole series by one number and adding a 

 constant. This usually gave the wave-lengths of the whole spectrum 

 within 0-1 or 0-2 divisions of Angstrom. The differences of this series 

 from the primary standards were then plotted, and a smooth curve 

 drawn through the points thus found. The ordinates of this curve 

 then gave the correction to be applied at any point. 



It is to be noted that the departure from the normal spectrum was 

 very small, and the correction thus found was very certain. The cause 

 of the departure was not apparent, but may have been the slight tilting 

 of the spectrum, by which it was measured somewhat obliquely at 

 places. 



The visible spectrum was thus gone over five or more times in this 

 manner, with several different gratings and in different orders of spectra. 

 The results are given in Table X, Columns C, R, p, q, m, 0, e, h, i, etc. 

 The spectrum from the green down to and including A was also ob- 

 served on a large instrument for flat gratings, having lenses six and 

 one-half inches in diameter and of eight feet focus. These latter 

 observations are marked C". This region I intend at some future time 

 to observe further. 



It was now required to observe the ultra violet to complete the series. 

 For this purpose the coincidences of the 3d, 3d, 4th, 5th, and 6th 

 spectra of a 7000, 21$ feet radius, grating were photographed. My in- 

 strument will take in photographic plates twenty inches long, but there 

 will be a slight departure from a normal spectrum in so long a plate. 

 Hence plates ten inches long were mostly used for this special series. 

 Before the camera was placed a revolving plate of metal about three- 

 sixteenths of an inch thick, and having a slit in it of the same width.' 



6 This is described in the Johns Hopkins Circular of May, 1889, by Dr. Ames. 



