26 BULLETIN OF THE 



One of them showed the well-known appearance of hexagons, the 

 other the longitudinal diffraction lines produced in the experiment 

 devised by Professor Abbe, of Jena. Professor Abbe had found 

 that if the central part of a high-angled objective be stopped out 

 by a transverse bar and a frustule of Pleurosigma angulatum exam- 

 ined, its midrib being parallel to the bar and the light thrown at 

 right angles to it, the hexagonal markings would be no longer seen, 

 but instead a series of sharp diffraction lines parallel to the mid- 

 rib. In their distance apart these markings are so related to the 

 distance of the hexagons from centre to centre that seven of the 

 diffraction lines occupy the space of four of the hexagons. By 

 lamplight. Professor Abbe found that these diffraction lines ap- 

 peared only on those parts of the Angulatum frustule which were 

 fused to the glass cover in preparing the specimen; where a film 

 of air existed between the frustule and the cover they did not 

 appear. Mr. Woodward had found that this was true only for 

 illumination by lamp ; by sunlight the diffraction lines appeared 

 on all parts of all the frustules. lu the photographs which he 

 exhibited the boundary of the adherent part of the frustule could 

 be distinctly seen, and the diffraction lines were equally distinct 

 on the non-adherent parts. Professor Abbe's explanation of the 

 phenomenon was therefore inadequate, as it required the non- 

 appearance of the diffraction lines on the non-adherent parts by 

 sunlight as well as by lamp. 



The subject was further discussed by Messrs. Harkness and 

 Woodward. 



Mr. Asaph Hall made a communication 



ON THE satellites OF SATURN. 

 (abstkact.) 



Mr. Hall stated that his observation of Hyperion, the faint 

 satellite of Saturn, made in 18t8, indicate that the line of apsides 

 of the orbit of this satellite has a rapid motion. Probably this 

 line has revolved at least 180° since the time of the discovery 

 of this satellite in 1848 by the Bonds and Lassell. 



Attention was called to the peculiar relations of the orbits of 

 Hyperion and of the large satellite Titan, through which Titan 

 is able to exert a great influence on the motion of Hyperion. 

 The motion of the line of apsides of Hyperion will probably fur- 

 nish an accurate determination of the mass of Titan. 



Remarks were made by Messrs. Taylor and Newoomb, 



