6 Oliver H. Gish 



tion at right angles to that of the bands. At first the glass wedge 

 was simply substituted in place of the mica plate, of the method 

 described above, and the slit of the spectroscope opened to a width 

 of about 4 mm. With the copper arc as a source, images of the 

 slit were distributed throughout the spectrum, each showing a set 

 of from three to six bands depending on the wave-length. Due 

 to the chromatism of the cjuartz lens only a limited portion of the 

 spectrum was in focus at a time, so to avoid this the glass plate 

 itself was diaphragmed down to an aperture of 4 by 10 mm. and 

 mounted in place of the slit of the spectroscope (Fig. 2). By 

 means of two quartz lenses (a and h) and a total reflecting cjuartz 

 prism («) an image of the arc was formed on this, the light being 

 incident as before at an angle ef about 2.5 degrees. When prop- 

 erly adjusted all the images of the set of bands were found well 

 defined. Photographic records were obtained upon heavy films of 

 such length as to include the first order spectrum and a large por- 

 tion of the ultra-violet region in the second order. The displace- 

 ment of the bands was measured with a micrometer microscope. 



Metliod of Measurement 



The difficulty of finding glass plates giving perfect bands was 

 one source of error. To eliminate as much as possible that arising 

 from the curvature of the bands, the cross line of the microscope 

 was first adjusted tangent to some band of one set, and measure- 

 ments of the shift of all the bands in the set then made. It was 

 then adjusted tangent to a band of the opposite set and the series 

 of readings again taken. Each of these series of readings was 

 repeated, the respective adjustments being on another band of each 

 of the two sets. The mean of these four sets of readings should be 

 practically free of the error arising from curvature, provided that 

 curvature is not abrupt and the points on the bands on which the 

 settings are made are not too far from the line of division of the 

 two sets. The first condition depends on the selection of the glass 

 plates ; the second on the care observed in making the settings. A 

 small region between the two sets of bands, owing to the astigma- 

 tism of the grating, was not well defined. This necessitated setting 

 on points that were somewhat removed from the real line of 



172 



