434 Prof. Powell's Further Remarks on Experiments 



Young, it is surprising that his lectures and writings did not 

 produce a general impression in favour of a branch of science, 

 which his researches had successfully reduced to a form not 

 less philosophically comprehensive, than experimentally simple 

 and striking. More lately Fraunhofer, in his elaborate re- 

 searches, has carefully pointed out the distinction that these 

 ^'■intervals" have a real existence, whatever theoretical view 

 we may take of their nature. The publication of Sir J. Her- 

 schel's Treatise on Light forms an epoch in the history of 

 the science, and has given a material impulse to the study of 

 it: and there are only wanting further endeavours to sim- 

 plify and facilitate the investigations, and to bring them more 

 within the reach of the generality of students, in order to dif- 

 fuse a knowledge of the subject as widely as its beauty and 

 importance demand. 



The only point which continued to be a matter of ques- 

 tion, viz. the application of interferences to the colours of 

 thin plates, having now been decisively settled by Professor 

 Airy's expcrimentum crucis (Phil. Mag. and Annals, August 

 1S31), the language of comparison between the theories which 

 was legitimately used before, has ceased to be admissible. 

 But the sagacity with which Newton detected the existence 

 of these intervals, as well as the accuracy with which he mea- 

 sured them, only continues to be enhanced in our estimation 

 by the more recent explanation of their nature. He observed 

 that at certain thicknesses no sensation of light reached the 

 eye: it was therefore a natural and unavoidable conclusion, 

 that none was reflected, — when as yet neither the paradoxical 

 fact that two reflexions might destroy each other, nor the 

 equally essential point that in this case there were two re- 

 flexions concerned, had been established. 



Next to the fact of the existence of the intervals, that of 

 their diminution in more refractive media is of fundamental 

 importance to the theory. The fact is indeed involved in a 

 variety of phasnomena : but as I have not happened to meet 

 with any detailed account of the method of showing it ex- 

 perimentally, except mixed up with other considerations, I 

 will here offer a few remarks upon the subject. 



In the first place it is evident, that if we employ the inclined 

 reflectors for the interference-stripes*, and they be immersed 

 in a transparent fluid, the surface of the liquid medium being 

 so arranged that the rays are incident perpendicularly, and if 

 the eye-lens be placed in such a manner as to receive the rays 



■ * The experiments, as well as the formula here referred to, are those de- 

 scribed in my former paper. — Pliil. Mag. and Annals, N.S. vol. xi. p. 4, &c. 



