LYMAN. — FALSE SPECTRA FROM THE ROWLAND GRATING. 245 



cealed by the strong real lines of the spectrum in places where strong 

 lines occur. They are chiefly noticeable in the extreme ultra-violet, 

 where there are no real lines. In parts of the spectrum where gaps 

 occur, however, these phantoms may be visible and may there be mis- 

 taken for real lines. Such, in brief, are the results arrived at. A 

 more detailed account of the apparatus and of the numerical results of 

 the investigation will now be given. The consideration of the causes 

 which produce these phantom lines is left for the end of the paper. 



Figure 1. 



The first grating investigated was one of 180 cm. radius. The plate 

 was prepared by Brashear and ruled by Rowland's engine in the year 

 1897 ; it had 14,438 lines to the inch. To economize space a mounting 

 somewhat different from that of Rowland was used. The grating and slit 

 were fixed in position at a distance apart equal to the radius of cur- 

 vature of the grating. The plate-holder was carried by an arm pivoted 

 at a point on the line connecting slit and grating and midway between 

 the two. The length of the arm was, of course, half the radius of 

 the grating. The plane of the grating was so adjusted that the prin- 

 cipal image fell about 1 cm. to the right of the slit. This was done to 

 avoid reflection from the polished jaws of the slit. The plate-holder was 

 constructed to take a plate five inches long and one inch broad. The 

 form of the box in which the apparatus was enclosed is not material to 

 this investigation. The spark which served as a source of light was 

 placed within 3 mm. of the fluorite window. In the first experi- 

 ments the width of the slit was about .1 mm., and the time of exposure 

 was about one hour. Later, however, good photographs were obtained 

 with exposures of from twelve to fifteen minutes. In making tests for 

 the absorption of the air a quartz lens was placed between the source 

 and the slit. 



In developing photographs of this type, great contrast is the chief 

 object. The kind of developer used thus becomes important. After 

 several trials, an Ortol-Soda developer was found to give the best results. 



The apparent wave lengths of the different lines which come under 

 consideration were determined by comparison with the normal sun spec- 



