ON LIGHT. 317 



therefore, proceed to the next branch of our general 

 subject, that of the Diffraction of Light. 



(101.) DIFFRACTION. The optical phsenomena which 

 refer themselves to this head are many and various. 

 They are not, for the most part, very obvious, but are 

 exceedingly curious and interesting in their details, and 

 some of them, under careful arrangement and with good 

 optical appliances, very brilliant. Familiar examples 

 offer themselves in the twinkling of the stars and the 

 changes of colour they exhibit during the different 

 phases of their scintillations : in the vivid radiating 

 streaks of light which seem to stream outwards from any 

 small and dazzlingly brilliant point of light (as for instance 

 the reflection of the sun on a small polished globe, as a 

 thermometer ball) : in the colours exhibited when a 

 bright point is seen reflected on or refracted through a 

 surface regularly striated or scratched across with fine 

 equidistant lines, as beautifully exhibited in the so-called 

 " Barton's buttons " (from the name of the ingenious and 

 skilful amateur mechanist who first executed them) ; 

 brass or steel buttons delicately cross-lined by engine 

 work : in the lateral images of a candle seen reflected 

 on polished mother-of pearl : and in the coloured halos 

 often seen to surround the flame of a candle in certain 

 states of the eye and their artificial imitations in a mode 

 presently to be described. Less obvious to common 

 observation, and requiring particular arrangement, in- 

 strumental or otherwise, to see them distinctly, are the 

 phenomena (referable to the head of diffraction) of the 

 rings and other appendages seen to surround the images 



