3l6 ON LIGHT. 



(100.) Connected with the colours of thin plates are 

 several distinct classes of optical phenomena, in which 

 colours of the same kind, and explicable on the same 

 principle, arise in the reflexion and transmission of light 

 through or between pellucid plates of considerable thick- 

 ness, or through spherical drops of water, examples of 

 which are to be observed in the pink and green fringes 

 which are often seen bordering the interior of a rainbow, 

 and in those similarly coloured fringes (of exceedingly 

 rare occurrence) which sometimes run, like a bordering 

 ribband, just within the contour of a thin white cloud 

 in the near neighbourhood of the sun. Upon this class 

 of phaenomena, however, we shall not dwell further than 

 to observe that they prove the law of the periodical re- 

 currence of similar phases at equal intervals, not to be 

 confined to very minute distances in the immediate 

 neighbourhood of reflecting or refracting surfaces, but to 

 extend over the whole course of a ray of light as, on 

 the undulatory theory, it necessarily must do. We now, 



whose knowledge of dynamics is limited by this very elementary 

 application. Properly speaking, we ought to assume the coer- 

 cive force to vary in the direct ratio of the distance; on which 

 supposition only will large and small vibrations be executed in 

 equal times. Calculating on this (the correct) principle, and taking 

 the extreme excursion (as in the text) at one-trillionth of an inch, 

 the ratio of the coercive force to gravity at that distance will be 

 found as 35,465,000,000 to I. On the other hand, as a strange 

 contrast to the immensity of such a force, we shall find the maxi- 

 mum velocity it will have generated on the arrival of the molecule 

 at the medial point of its vibration not to exceed i-27Oth part of an 

 inch per second I 



