THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



OCTOBER 1879. 



XXXI. Investigations in Optics, with special reference to the 

 Spectroscope. By Lord Rayleigh, F.R.S.* 



[Plate VII.] 



§ 1. Resolving, or Separating, Power of Optical Instruments. 



ACCORDING to the principles of common optics, there is 

 no limit to resolving-power, nor any reason why an 

 object, sufficiently well lighted, should be better seen with a 

 large telescope than with a small one. In order to explain 

 the peculiar advantage of large instruments, it is necessary to 

 discard what may be looked upon as the fundamental principle 

 of common optics, viz. the assumed infinitesimal character of 

 the wave-length of light. It is probably for this reason that 

 the subject of the present section is so little understood out- 

 side the circles of practical astronomers and mathematical 

 physicists. 



It is a simple consequence of Huyghens's principle, that the 

 direction of a beam of limited width is to a certain extent in- 

 definite. Consider the case of parallel light incident perpen- 

 dicularly upon an infinite screen, in which is cut a circular 

 aperture. According to the principle, the various points of 

 the aperture may be regarded as secondary sources emitting 

 synchronous vibrations. In the direction of original propa- 

 gation the secondary vibrations are all in the same phase, and 

 hence the intensity is as great as possible. In other clirec- 



* Communicated by the Author. 

 Fhil. Mag. S. 5. Vol. 8. No. 49. Oct. 1879. T 



