Dr. Gladstone on the Electric Light of Mercury, 251 



so on. Fig. 2 represents the principal dark lines of the solar 

 spectrum, as seen by means of the same prism, and is introduced 

 to identify the positions of the luminous rays in fig. 1. The 

 following Table gives the angular measurements of the more im- 

 portant rays, those of Fraunhofer's lines being appended in a 

 separate Table : — 



Whether the thin stream of mercury was longer or shorter, 

 whether the light was steady or intermittent, and whether it was 

 more or less brilliant, the relative intensity of these luminous 

 rays appeared the same, except as regards the most refrangible 

 ray, which varied considerably in visibility. This ray is situated 

 far beyond what is ordinarily considered the limit of the lumi- 

 nous spectrum, indeed at the utmost verge of the most refran- 

 gible light which I have been able to see with bright midsummer 

 sunshine and special arrangements for detecting it. It is evi- 

 dent, therefore, that this ray exists in the electric light of mer- 

 cury with an intensity vastly greater than it exhibits in the solar 

 beam ; and doubtless many objects (as cochineal) reflect or trans- 

 mit a colour when examined by this light, with which, under 

 other circumstances, we are unacquainted. It should be borne 

 in mind, moreover, that this ray, as observed through the refrac- 

 tion goniometer, had passed through several pieces of glass — a 

 medium which does not transmit freely the extra-violet rays. As 

 to its colour, it seems to differ considerably according to the in- 

 tensity ; and the eye is perhaps not a good judge of a colour 

 which, strictly speaking, it has never witnessed before. "When 

 bright, however, it may be best described perhaps as red- violet ; 



S2 



