532 SCIENCE PROGRESS 



has now actually come to pass, and we have to place on record 

 the first direct measurement of the angular diameter of a star. 

 Before dealing with the actual observation, it is of interest 

 to note that the possibility of measuring the angular diameter 

 of a star in this way was first pointed out by the French 

 physicist Fizeau about fifty years ago. It provides a good 

 illustration of an incidental suggestion, thrown out more or less 

 casually, subsequently bearing fruit. Fizeau's remark occurs 

 towards the end of a report on the award of the Bordin Prize 

 of the Academic des Sciences, and is as follows : 



" II existe, pour la plupart des phenomenes d 'interference, 

 tels que les f ranges d 'Young, celles des miroirs de Fresnel, et 

 celles qui donnent lieu a la scintillation d'apres Arago, une 

 relation remarquable et n^cessaire entre la dimension des 

 franges et celles de la source lumineuse ; en sorte que les franges, 

 d'une tenuite extreme, ne peuvent prendre naissance que lorsque 

 la source lumineuse n'a plus que des dimensions angulaires 

 presque insensibles ; d'oii pour le dire en passant, il est peut- 

 etre permis d'esperer qu'en s'appuyant sur ce principe et en 

 formant, par exemple au moyen de deux larges fentes tres- 

 ecart^es, des franges d 'interference au foyer des grands in- 

 struments destines a observer les 6toiles, il deviendra possible 

 d'obtenir quelques donnees nouvelles sur les diametres angu- 

 laires de ces astres " {Comptes Rendus, 66, 934, 1868). 



The suggestion of Fizeau was investigated practically by 

 St^phan, who worked out an approximate theory of the 

 phenomenon. He extended the suggestion by proving that, 

 instead of narrow slits, it was permissible to use extended 

 apertures, provided that the two apertures were equal and 

 possessed two axes of symmetry and that their width was 

 small compared with their distance apart. In this way much 

 more light could be obtained than with narrow slits, making 

 the observations easier. St^phan used the 80-cm. refractor at 

 the Marseilles Observatory and examined Sirius and other stars, 

 which he anticipated would have a relatively large apparent 

 diameter. Diameters not less than o''*i6 should have been 

 measurable with the instrument, but Stephan could not find 

 any star for which the fringes would vanish, and from their 

 appearance he concluded in a paper entitled " Sur I'extreme 

 petitesse du diametre apparent des 6toiles fixes " {Comptes 

 Rendus, 78, 1008, 1874) that " les experiences citees ne prouvent 

 pas seulement que le diametre apparent des ^toiles examinees 

 est inf^rieur ^ o'''i6. Elles montrent encore que ce diametre 

 est une tr^s faible fraction du nombre precedent." We now 

 know from theoretical considerations that for few, if any, stars 

 do the apparent diameters exceed o^'OS, and for most are much 

 inferior to this limit, so that Stephan 's conclusion was correct 



