50 J: Intelligence and Miscellaneous Articles. 



The method has been experimentally tested by Stephan (1873), 

 who made use of the large telescope of the Marseille Observatory. 

 On placing in front of the object-glass a lunular screen, which 

 allowed the light to pass through the two opposite borders, he 

 was enabled to infer the existence of an apparent diameter of 

 0"-2. In fact, the stars of the 1st, 2nd, and 3rd magnitudes 

 observed have an apparent diameter inferior to this limit. 



Since then. M. Michelson (about 1892) has made a careful study 

 of the theory of these experiments, and has verified all its conse- 

 quences, by first using terrestrial sources of various dimensions. 

 Further, using a 12-iuch equatorial, he has measured the diameters 

 of Jupiter's satellites. 



M. Maurice Hamy has taken up the study of this subject, with 

 a view to substituting large slits, which allow a great deal of light 

 to pass through, for the very narrow apertures employed in the 

 previous researches. He has arrived at a formula which solves the 

 problem. Experiments have been carried out with the large equa- 

 torial of the Paris Observatory. The fir?t measurements were 

 made under unfavourable atmospheric conditions (on June and 

 July nights). Nevertheless, their results are very concordant. 

 The differences from day to day are, in general, less than one- 

 tenth of a second. On rejecting the highest numbers, which 

 correspond to an increase in the apparent diameter due to irregular 

 atmospheric refraction, the definite results for Jupiter's satellites 

 are respectively 



I. II. I IV. 



0"-98 0"-87 l"'28 1"*31 



Michelson's results are : — 



l"-02 0"-94 l'-37 1"-31 



The agreement between the two sets of values is remarkable. 



M. Maurice Hamy has applied the same method to the measure- 

 ment of the apparent diameter of the planet Vesta, and finds for 

 it 0"-54. 



Now this is precisely the number found by Barnard by means 

 of direct micrometric measurements made with the enormous 

 telescope of the Lick Observatory ; the interference method fur- 

 nishes the same result with much greater ease. 



The author concludes by showiug that, in spite of its great sen- 

 sitiveness, the interference method is powerless so far as the 

 determination of the apparent diameters of stars is concerned. If, 

 in fact, the sun were transported to a distance equal to that of the 

 star a Centauri, its apparent diameter would tall below the one- 

 hundredth of a second ; and in order to determine this, even with 

 the interference method, the telescope would have to be move than 

 12 metres in diameter! 



One may, it is true, imagine an interference apparatus which is 

 applicable, theoretically, to the study of the diameters of stars and 

 which does not require a telescope of extraordinary dimensions ; but 

 the experimental difficulties connected with it are so great that 

 they do not appear to be capable of being surmounted. — Seances de 

 la Societe Francaise de Physique, Feb. 16th, 1900. 



