18 BULLETIN OF THE 



The minutes of the last meeting were read and adopted. 



The election of Maj. William J. Twining, of XT. S. Engineers, 

 and Mr. David P. Todd, of the Nautical Almanac Office, as 

 members of the Society, was announced. 



A paper by Mr. B. S. Holden, entitled 



NOTES ON THE BRIGHTNESS AND THE STELLAR MAGNITUDE OF THE 

 THIRD SATURNIAN SATELLITE, TETHYS, 



was read by Mr. Skinner, 



Remarks were made by Mr. Abbe. 

 Mr. J. J. Woodward made remarks 



ON the APERTOMETER OF PROF. E. ABBE, OF JENA, GERMANY. 

 ( A B STR A CT.) 



Mr, Woodward exhibited and described the apertometer de- 

 vised by Professor Abbe, of Jena, for measuring the aperture 

 of microscopic objectives, and manufactured by Carl Zeiss, also 

 of Jena. An account of this instrument will be found in the 

 Journal of the Royal Microscopical Society, March, 1878, p. 19. 

 As the instrument was designed for use on the perpendicular 

 microscope stands commonly employed in Germany, Mr. Wood- 

 ward had modified it by mounting the glass disk on a block of 

 wood 3 inches long by 1.5 wide and .4 thick, and by attaching 

 springs of thin sheet brass to the two movable indices, the object 

 being to allow the apparatus to be used on English or American 

 stands inclined at convenient angles. He praised the ingenuity 

 of Professor Abbe's device and the excellence of the workman- 

 ship of Zeiss, and commended the instrument as affording a con- 

 venient method of measuring the apertures of objectives by lamp- 

 light. He still preferred, however, as simpler and more convenient, 

 the method he had used for several years in measuring such aper- 

 tures by sunlight. As originally used, this method was described 

 by him in the Monthly Microscopical Journal, June, 1873, p, 

 208. The objective was screwed to an opening in the shutter of 

 the dark room; a parallel pencil of solar light was thrown through 

 it from behind by a plane mirror, while the front lens of the ob- 

 jective was in contact with a thin sheet of Canada balsam confined 

 between two plates of glass, one of them ground. The solar 

 rays, after coming to a focus in the balsam, diverged, and the 

 margins of the cone of light could readily bo marked with a lead- 

 pencil on the surface of the plate of ground glass. The angle 

 formed was afterwards measured with* a protractor, which gave 



