416 Prof. Plucker on the Electrical Discharge 



as from D ; finally, a beautiful yellow band, E. Of the three 

 bands in the green, D is especially bright, being of a yellowish 

 tinge; then follows C in respect of intensity, and finally B. 

 The red is very prominent, spreading over an extensive surface. 

 A well-defined thick black line occurs near its further boundary. 

 On the other side it is sharply separated from the yellow band 

 E by a broad black line : the whole of the yellow light is con- 

 centrated upon this band. The band D is separated from the 

 band E by a grey interval, almost equal in breadth to that 

 separating D and C. The violet light is confined to the band 

 A. The space between A and B is divided into a completely 

 black space, and one of an indefinite dark colour. The black 

 space sharply bounding band A, is about three times as broad 

 as each of the five bright equally broad bands, and about a 

 third of the breadth of the whole space between A and B, or 

 between B and D. 



71. As another example, I shall take the beautiful spec- 

 trum of gaseous fluoride of boron as one totally different from 

 that just described. In this spectrum, which shows bright 

 colours from one extremity to the other, red, together with 

 orange and yellow, takes up about a fifth of the whole space ; of 

 the other four-fifths, two are occupied by the green, and the 

 remainder by the violet. It is remarkable that the eye does 

 not perceive any transition between the violet and the green, 

 the blue appearing to be entirely wanting. Yellow and orange 

 form two sharply bounded bright bands of about equal breadth, 

 both together being about half as wide as the red, from which 

 the orange is separated by a strong black line. Near the 

 boundary of the red there is a second such black line. The 

 space of the red between these two black lines is divided into 

 six equal parts by five fine black lines ; and on the other side of 

 the strong black line first mentioned, there is in the orange and 

 yellow also the same number of fine black lines at the same 

 distance from one another. In the green, about twice as far 

 from the violet as from the yellow boundary, there is a bright 

 green band about as broad as the yellow band. This green 

 band divides the green space into two parts, which difi"er im- 

 portantly from one another. That part which lies next to the 

 yellow, has in the middle a bright shining band ; and the green 

 on either side of this band is not uniform, for it becomes con- 

 tinually darker towards its two extremities. The remaining 

 portions of the green and of the violet have quite a charac- 

 teristic appearance. This space appears divided into sixteen 

 bands, ten of which belong to the violet. Each separate band 

 is bright towards the red, becoming gradually darker in the 

 opposite direction. The breadth of these bands is not quite 



