502 Prof. E. Wartniann on some New Lines 



the widest aperture, the longitudinal with the smallest, and 

 the two systems with the mean one. With a constant aper- 

 ture the vertical lines appear at the greatest distance, the ho- 

 rizontal at the smallest, and the double system at the mean 

 distance. The angle of incidence for the transversal lines 

 alone, is a little larger than that which gives a distinct vision 

 of the longitudinal alone. The arrangement of the longitu- 

 dinal lines varies with the distance of the prism from the 

 aperture. The first transversal as well as longitudinal lines 

 which begin to appear in the field of the solar spectrum, 

 are those which, in the entire system, are the most intense 

 and the widest. In each system the focus of the lines varies 

 with the size of the aperture. The intensity of the lumi- 

 nous lines in each of the three systems is greater than that of 

 the remainder of the field of the solar spectrum : this is what 

 Fraunhofer discovered for the transversal lines. The state of 

 the atmosphere modifies the focal distance of the longitudinal 

 as well as of the transversal lines. This state also influences 

 the position, number, size and force of the longitudinal and 

 transversal lines. To each system of lines corresponds an 

 aperture, which is best suited for its being projected most 

 distinctly. 



" Fraunhofer refers the cause of the black lines to the nature 

 of the light. With him, the spectrum was a means of verifying 

 the identity or diversity of the light emanating from different 

 sources. Herschel ascribes the origin of it to the positive 

 absence of luminous rays, whether in the act of their develop- 

 ment, or by absorption in the media which they have to pass 

 through. This opinion is very nearly the same as that enter- 

 tained by Brewster and Erman. The former considers that 

 the rays which are defective are absorbed by the gases pro- 

 duced in the combustion which engenders the light, or by the 

 media through which they must pass. The latter is of opinion 

 that the medium traversed by the luminous ray separates it 

 into two or into several parts, each of which is retarded in a 

 different manner ; on this account he distinguishes simple and 

 double interference. 



" All these opinions appear to me to be defective, although 

 they all seem to have a true side. Fraunhofer refers the entire 

 phajnomenon to the nature of the luminous ray alone ; Her- 

 schel, Brewster, and Erman to the simple influence of the 

 different media interposed. But in these phaenomena, the 

 apparatus itself which is used for the experiment plays an 

 important part. It is now a well-proved fact, that the appear- 

 ances of the spectrum vary with the size of the aperture and 

 the interval which separates it from the prism, the nature and 



