Mr. Gassiot on a Train of Eleven Sulphide-of-carbon Prisms. 71 



metal framing of the prisms, and the various adjusting-screws, have 

 been omitted in the last sketch. 



_ The very unfavourable state of the weather prevented any observa- 

 tions being made on the solar spectrum with these prisms until 

 Saturday the 12th inst. The results then obtained may probably 

 not be considered devoid of interest. They are as follows : — 



The prisms were arranged so as to enable that portion of the spec- 

 trum to be observed in which the well-defined D line of Fraunhofer 

 is situated. This line, long since resolved as double, presented an 

 angular separation of 3' 6", measured from the centre of one to that 

 of the other principal line, this measurement being made by Mr. 

 Balfour Stewart by means of the micrometer attached to the tele- 

 scope ; the value of the divisions of the micrometer he had previously 

 determined relatively to the divided circle of the spectroscope. A 

 centre line (clearly defined and figured in Kirchhoff and Bunsen's 

 map) was distinctly visible, and nearly equidistant from the centre 

 towards the violet ; five clearly defined lines were perceptible, as also 

 two faint lines on each side of the principal lines, between the centre 

 line of Kirchhoff towards the red. Several faint lines were also per- 

 ceptible. 



The lines as represented in the diagram were drawn by Mr. 

 Whipple, one of the assistants in the Observatory, as they were 

 observed by him about 3.45 p.m Some of these may possibly be 

 due to the earth's atmosphere, but the five most refrangible lines 

 were observed at an earlier period of the day by Mr. Stewart, Mr. 

 Browning, and myself. 



The great angular separation of the double D line to 3' 6" is a 

 proof of the power of this arrangement of the sulphide-of-carbon 

 prisms, and offers the means of mapping out the entire solar spectrum 

 on a scale not hitherto attained. 



Note. — Since the preceding observations were recorded, an inspec- 

 tion has been made of the region of the spectrum towards the refran- 

 gible side of double D ; and, from the comparisons made with a map 

 of lines obtained by means of the battery of glass prisms with that 

 given by those of the sulphide-of-carbon prisms, many new lines are 

 produced in addition to those observable by the former, while the 

 battery of glass prisms itself gives a number of additional lines to 

 those that are depicted in Kirchhoff' s map. 



