Theory of Nobili's Coloured Rings. 



15 



of the current, as where the differences of the radii are great, 

 or by a defective determination of the film of peroxide of lead, 

 as when different systems of rings are used, which will be 

 mentioned more in detail below. A line was drawn from the 

 centre upon the plate covered with the coloured rings, and this 

 radius successively placed in the portions of a prismatic solar 

 spectrum, which are indicated by the lines B, D, E, and F. 

 The following table contains the results of the experiments 

 with two different plates. The second system of rings was used 

 in each, this being the most perfectly formed. To render the 

 table intelligible, it need only be remarked, that r denotes the 

 radius in each case, and r b the radius of the ring, which was 

 measured when illuminated by the spot B of the spectrum. 



Table I. 



Table II. 



The length of the waves of the various colours was deter- 

 mined by assuming as a basis for the red at the line B, the 

 length given by Schwerd, viz. 0000688 millimetres; this num- 

 ber was multiplied by the proportion of the thickness, i. e. 

 by the inverse cubic proportion of the radii for the ring of 

 the red, and that of the colour observed each time. The 

 numbers used by M. E. Becquerel, deduced from FresnePs 

 experiments, differ considerably from those of Schwerd, 

 as m 0006&5 millim. is given as the length of the waves in 

 the red at the line B, which however corresponds to a red 

 between C and D. The results obtained show unequivo- 



