Spectral Lines of Solar and Terrestrial Origin. 461 



of one of the extremities for the other produces the double 

 displacement of the solar radiations, while it has no influence 

 upon the position of the absorption-lines of the atmosphere. 



The analysis of the optical conditions shows that perfect 

 sharpness of the displacement can only be obtained when three 

 experimental conditions are fulfilled : — 



1. The spectral images at the focus of the telescope of the 

 spectroscope must be aplanatic ; that is, such that the vertical 

 lines (spectral lines) and the horizontal lines (due to the im- 

 perfections of the slit) are equally sharp in the same plane. 



2. This plane must coincide accurately with that of the 

 cross wires. 



3. The focal image of the solar disk must be exactly in the 

 plane of the slit of the collimator. 



The omission of any one of these adjustments would involve 

 an abnormal displacement of the lines consequent upon a dis- 

 placement of the collecting-lens. On the other hand, when 

 all these adjustments have been made the dark lines of terres- 

 trial origin are perfectly fixed, while those of solar origin 

 move with extreme sharpness. 



If we take as a fixed mark a dust particle on one of the cross 

 wires, such as are always to be found on wires, we are able 

 to distinguish immediately, at a glance, solar from telluric rays. 



Finally, I may add that the displacement becomes still 

 more sensible when the substitution of one solar edge for the 

 other is effected rhythmically. This is done by causing the 

 collecting-lens to oscillate two or three times per second *. 



* The following (see fig. 2) is the arrangement employed : — The col- 

 lecting-lens C is carried by a socket S S' which rests on the table U U' of 



Fig. 2. 



the spectroscope by two points P (one lies behind the other in the figure), 

 about which it can turn. The optic centre C of the collecting-lens 

 describes a small horizontal element, and transmits the same motion to 

 the image of the solar disk. The observer produces this motion by the 

 aid of the lever L. The pinion H serves to put the collecting-lens exactly 

 in focus on the slit. 



