THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



JULY 1890. 



I. On the Application of Interference Methods to Astro- 

 nomical Measurements. By Albert A. Michelson*. 



[Plates I. & LL] 



IN a recent paper on " Measurement by Light-Waves " f it 

 was shown that the limitation of the effective portions of 

 an objective to the extreme ends of a diameter converted the 

 instrument into a refractometer ; and although definition 

 and resolution are thereby sacrificed, the accuracy may be 

 increased ten to fifty fold. 



The simplest way of effecting this in the case of a telescope 

 is to provide the cap of the objective with two slits adjustable 

 in width and distance apart. If such a combination be focused 

 on a star, then, instead of an image of the star, there will be a 

 series of coloured interference-bands with white centre, the 

 bands being arranged at equal distances apart and parallel to 

 the two slits. The position of the central white fringe can be 

 marked from ten to fifty times as accurately as can the centre 

 of the telescopic image of the star. 



One of the most promising applications of the method is the 

 measurement of the angular magnitudes of small sources of 

 light. 



This may be accomplished by taking advantage of the well- 

 known principle that in order to obtain clear interference- 



* Communicated by the Author. 



f ' American Journal of Science,' xxxix. Feb. 1890. 



Phil. Mag. S. 5. Vol. 30. No. 182. July 1890. B 



