DISPERSION. 43 



(52) On a minute examination of the solar spectrum, 

 when every care has been taken in making the experiment, 

 it is found that it is not, as Newton supposed, a continuous 

 band of coloured light, whose intensity is greatest about the 

 confines of the yellow and orange, and diminishes regularly 

 to the two extremities ; but that, on the contrary, there are 

 at certain points abrupt deficiencies of light, total or partial, 

 indicated by the existence of numerous dark lines or bands, 

 crossing the spectrum in the direction at right angles to its 

 sides ; while in the intermediate spaces the intensity of the 

 light does not increase or decrease continually, but varies 

 irregularly, or according to some very complex law. Solar 

 light, then, does not consist of rays of every possible refran- 

 gibility, within certain limits, for it is found that many rays 

 corresponding to certain degrees of refrangibility are wanting 

 in the spectrum. 



Some of these lines are wholly black; others dark, of 

 various degrees of illumination. Again, some of them are 

 well defined and single ; others are clustered together, so as 

 to present the appearance of dark bands. They are irregu- 

 larly disposed throughout the length of the spectrum. They 

 are not, however, the result of any accidental cause ; for, when 

 solar light is used, and the refracting substance is the same, 

 it is found that they preserve the same relative position, both 

 with respect to one another and to the colours of the spec- 

 trum. On the other hand, when the refracting substance is 

 varied, their relative positions with respect to one another are 

 altered ; but their positions as referred to the colours of t]ie 

 spectrum, as also their relative breadth and intensity, remain 

 unchanged. 



(53) If other kinds of light as that of the fixed stars, 

 flames, the electric spark are examined in the same way, 

 similar bands are discovered, but differing in each species of 



