116 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



that of the frontal lens. It will resolve, with, axial light, Nobert's twelfth) 

 group (2830 lines to the mil). 



(4) Objective 4 mm., N.A. 0-95 ; and 3 mm., N.A. 0-95 (fig. 16).— 

 These resemble the last in design. Dr. van Heurck finds that the 

 3-mm. will resolve Nobert's thirteenth group, and considers it somewhat 

 superior to the 4 mm. 



(5) Objective 2* 5 mm., N.A. 1*25, water-immersion (fig. 17). — Con- 

 struction like last ; a very beautiful objective. A slight alteration of. 



Fig. 16. 



Fig. 17. 



Fig. 18. 



diaphragm produces a marked effect on the image, and with axial light 

 the resolution lies between Nobert's 13th and 14th groups. With electric- 

 oblique light Nobert's 18th group is distinctly resolved. 



(6) Objective 3 mm., N.A. 1*40, homogeneous (fig. 18). — This con- 

 tains ten lenses. The simple frontal lens slightly exceeds a hemisphere,. 



Fig. 19. 



and the object of such a lens is to produce a notable amplification of the ■ 

 object without introducing at the same time great chromatic and sphe- 

 rical aberrations. It reduces the pencil aperture from 1*40 to 0-65. The- 

 first doublet discharges similar functions ; at first it diminishes the 

 pencil divergence, and afterwards intentionally introduces a certain- 



