288 ON LIGHT. 



ticular colour assuming a greater richness and depth of 

 tint, but, by the gradual withdraival of the faint tints 

 from the vertex, while fresh, and more and more in- 

 tense hues appear at that point, and open out into cir- 

 cular rings surrounding it ; giving place as they enlarge 

 to others still more brilliant, until at length a very bright 

 white spot makes its appearance, quickly succeeded by 

 a perfectly black one. Soon after the appearance of 

 this the bubble bursts. During the whole process it has 

 been growing gradually thinner by the slow descent ot 

 its liquid substance on all sides from the vertex, till at 

 length the cohesion of the film at that point gives way 

 under the general tension of the surface. The annular 

 arrangement of the colours, and the coincidence of their 

 common centre with this, the thinnest point of the film, 

 evidently go to connect their tints with the tliicknesses 

 of that film at their points of manifestation, and to indi- 

 cate that a certain tint is developed at a certain thickness^ 

 and at no other. This, we shall presently see, is really 

 the case. 



(71.) The order of the colours and the sequence of the 

 tints is in all cases one and the same, provided the series 

 be complete, i.e.^ provided time has been given for the 

 black central spot to form. Thus the first series, or 

 order, contained within the first ring consists of black, 

 very pale blue, brilliant white, very pale yellow, orange, 

 red ; the second of dark purple, blue, imperfect yellow- 

 green, bright yellow, crimson ; the third of purple, blue, 

 grass green, fine yellow, pink, crimson; the fourth of 

 bluish-green, pale pink inclining to yellow, red ; the 



