8 KENT AND TAYLOR. 



standpoint, considering the primary line of width approximately 

 0.25 t. m., and not as formerly, one of the grid components itself. 



These components are indeed, in this sense, each narrower than the 

 'primary maximum — 0.25 t. m. — the grid components, all of them 

 now regarded as secondary maxima, being only about 0.05 t. m. in 

 width in echelon No. 2. 



(2) The curvature of one of the mercury yellow lines was compared 

 with that of a grid component in X6104. By stopping down the, 

 echelon spectroscope slit, a line of definite length was observed, and 

 by setting the stationary cross-hair of the filar micrometer upon 

 the ends of the image, and the movable system upon its center, the 

 horizontal distance, d. Figure 2, from the ends of each line to its 

 center were measured. It was found that the curvature of the com- 

 ponent is about 25% greater than that of the primary line. 



(3) With a small mirror, set at 45°, over the lower half of the echelon 

 spectroscope slit an argon vacuum tube and the lithium arc were 

 observed at the same time. The relative motion of the grid compo- 

 nents in X6104 and a nearby argon line were then studied as the 

 echelon was rotated. The primary argon line moves about one-half as 

 fast as the grid components. 



Quantitative measurements of the relative displacements were later 

 made with Zn X4810. A quartz vacuum tube was fitted with coiled 

 brass wire leads and brass terminals, exhausted, filled with hydrogen' 

 to 10 or more cm. pressure and then gradually exhausted to 1 mm. or 

 less. The zinc liiles given by the brass wire leads appeared very 

 sharp, steady and brilliant. With X4810, as thus produced, was com- 

 pared the "gridded" line of a cored carbon arc at atmospheric pres- 

 sure, in which small pieces of zinc had been placed, the small mirror 

 arrangement allowing simultaneous observation of both sources. 

 Upon rotation of the echelon the grid components rushed by the 

 narrow tube line. To measure the relative speed a plane mirror was 

 attached to a side of the echelon case. The image of an illuminated 

 slit in a piece of cardboard was formed by a lens upon a distant scale 

 after reflection from the mirror. The echelon was set near the 6 = 

 position. A reading of the position of the slit image on the scale was 

 taken when the tube line lay upon the fixed hair of the filar micrometer. 

 The echelon was then rotated until the slit image moved about 2 cm. 

 The displacement of the tube line was then measured by the movable 

 cross-hair system. A similar series was then taken with a grid line. 

 The ratio of the displacements was 3.6 : 6.4 or about 1 : 2. 



(4) The echelon w^s rernoved and the ocular focussed on the prism 



