409 



Having ascertained the chromatic relations between sixteen colours 

 selected from the spectrum, the next step is to ascertain the positions 

 of these colours with reference to Fraunhofer's lines. This is done 

 by admitting light into the shorter arm of the instrument through 

 ther slit which forms the eyehole in the former experiments. A pure 

 spectrum is then seen at the other end, arid the position of the fixed 

 lines read off on the graduated scale. In order to determine the wave- 

 lengths of each kind of light, the incident light was first reflected 

 from a stratum of air too thick to exhibit the colours of Newton's 

 rings. The spectrum then exhibited a series of dark bands, at 

 intervals increasing from the red to the violet. The wave-lengths 

 corresponding to these form a series of submultiples of the retarda- 

 tion ; and by counting the bands between two of the fixed lines, whose 

 wave-lengths have been determined by Fraunhofer, the wave-lengths 

 corresponding to all the bands may be calculated ; and as there are 

 a great number of bands, the wave-lengths become known at a great 

 many different points. 



In this way the wave-lengths of the colours compared may be 

 ascertained, and the results obtained by one observer rendered 

 comparable with those obtained by another, with different apparatus. 

 A portable apparatus, similar to one exhibited to the British 

 Association in 1856, is now being constructed in order to obtain 

 observations made by eyes of different qualities, especially those 

 whose vision is dichromic. 



II. " On the Insulating Properties of Gutta Percha." By 

 FLEEMING JENKIN, Esq. Communicated by Professor 

 WILLIAM THOMSON. Received February 9, 1860. 



(Abstract.) 



The experiments described in this paper were undertaken with the 

 view of determining the resistance opposed by the gutta-percha 

 coating of submarine cables at various temperatures to the passage 

 of an electric current. 



The experiments were made at the works of R. S. Newall and Co., 

 Birkenhead. The relative resistance of the gutta percha at various 

 temperatures was determined by measuring the loss on short lengths 



