PHILOSOPHICAL SOCIETY OF WASHINGTON 25 



Remarks were made by Messrs. Haukness, Elliott, and 

 Taylor. 



Mr. J. J. Woodward also made a communication 



ON THE oil-immersion OBJECTIVES OF ZEISS, AND ON CONVENIENT 

 METHODS OF OBTAINING OBLIQUE ILLUMINATION FOR THESE AND 

 SIMILAR OBJECTIVES. 



(abstkact.) 



Mr. Woodward exliibited and described his snb-stage prism 

 for the illumination of balsam mounted objects for examination 

 with immersion objectives whose balsam angle (or preferably 

 interior angle) is 90° or upwards. It is described in the Journal 

 ■of the Royal Microscopical Society, November, 1878, p. 246. 

 By using a prism of 98' angle instead of a right-angled one, 

 immersion objectives of less than 90° but more than 82° interior 

 angle could be satisfactorily illuminated. Both prisms bad re- 

 cently been manufactured without any mounting by G. S. Wool- 

 man, 116 Fulton Street, New York. This had the advantage of 

 economy; but the prism being necessarily attached to the under 

 surface of the slide by glycerine or oil of cloves, would move with 

 the object, which was inconvenient. He therefore greatly pre- 

 ferred the instrument as originally made, although of course the 

 same results could be obtained by Woolman's modification, pro- 

 vided the prism was readjusted each time the object was moved. 

 He also exhibited and described a more complex apparatus which 

 he had devised for microphotography, and which enabled him to 

 register the precise angle of the illuminating pencil employed. 

 {This apparatus will be described in full and figured in the Jour- 

 nal of the Royal Microscopical Society during 1879.) 



Mr. Woodward remarked that although the most oblique pen- 

 cil an immersion objective can take in is necessary to obtain the 

 best results on lined test objects, e. g., Aniphipleura pellucida 

 mounted in Canada balsam, this was no longer the case when 

 direct sunlight was used. In illustration he exhibited a photo- 

 graph of Amphipleura pellucida taken with about 2700 diameters 

 by the oil-immersion -j^^^th of Zeiss, referred to by him in his 

 communication of November 23d, when he stated that its interior 

 ■angle was 114°. The photographs excelled in definition any 

 picture of this test he had ever obtained with any objective; and 

 yet the optical axis of the illuminating lens (a 3-inch objective of 

 10° aperture which was used without any substage lens or prism) 

 was only inclined at an angle of 45° to the optical axis of the 

 microscope. As a further illustration of the difference between 

 the results obtained by illuminating the microscope by lamp and 

 sunlight, he exhibited two photographs of Pleurosigma angulatum 

 taken by the oil-immersion ^ih of Zeiss of 115° interior angle. 



