366 



can be seen most easily and most distinctly in the place where the 

 bows are, but they absolutely cannot be seen anywhere else. 



The parallelism also of the streaks to the bows, in the author's 

 estimation, proves that the same cause which determines the direc- 

 tion of the bow, must determine that of the streaks, and thus esta- 

 blishes their dependence on critical separation. 



Dr. Herschel also contends, that streaks of different colours could 

 not be produced by a plate of air, of uniform thinness, between plain 

 surfaces, and that the prevalence of a blue colour in the streaks be- 

 longing to the blue bow, and of the converse in those belonging to 

 the red bow, prove their dependence on critical separation. 



Since it has been conceived by other persons, that by means of a 

 plateof air, havingtheform of an extremely thin wedge, straight bands 

 of colour would be produced between plain surfaces slightly inclined 

 to each other ; and as an experiment in support of this opinion had 

 been shown to the author, he gives his own explanation of the fact : 

 and he ascribes the production of the colours to distortion of the sur- 

 faces, because a degree of force was in that instance employed for 

 the purpose of producing the requisite contact at one extremity of 

 the glass. And since in other experiments, made with perfectly 

 plain surfaces, where no pressure was employed no streaks could 

 be seen, Dr. Herschel concludes that when streaks are seen, the sur- 

 faces employed are either not plain in their general extent, or are 

 terminated by some inconceivably small curvature at the edges in 

 contact. 



It has, in the next place, been observed to the author, that in the 

 enlarged figure which he has given in his last paper to illustrate the 

 streaks, the vacancies observable correspond with, and depend upon, 

 the assumed intervals between the rays, which in that figure are re- 

 presented as originally separated by blank spaces. 



Dr. Herschel admits that there is some plausibility in this objection, 

 but informs us, that the sxipposed force of it is founded on a miscon- 

 ception of the figure, which is not designed to represent the visible 

 arrangement and colours of the streaks, which can only be deduced 

 from their mixture at the place where they enter the eye ; but he de- 

 clines a thorough investigation of this point, because it would really 

 be an endless undertaking. 



One section of the present communication is devoted to the con- 

 sideration of the breadth of the streaks compared to that of the bows, 

 and the cause why they must take up a broader space than the bows 

 from which they are derived ; because it has been remarked, that this 

 circumstance precludes the possibility of accounting for them by cri- 

 tical separation. But although this remark may at first view appear 

 to be justified, it must be remembered (says the author) that the mo- 

 difying power of the surfaces is added to the principle of the critical 

 separation. The modification specifically named by the author, is 

 that of reflection by the plain surface held under the prism, which, 

 in the first instance, magnified the extent to 2 times the breadth of 

 the bow ; and if the reflection be repeated any number of times be- 



