ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



115 



mend it, when compared with the other here illustrated. A considerable 

 portion of the extra two pounds weight seems to be for the purpose of 

 making the stand firm on its feet, which it fails to do. In the use of 

 Microscopes it has been my need to frequently lift them from one place 

 to another, sometimes with considerable speed. Perhaps the extra two 

 pounds may have been added partially with the view of strengthening 

 the muscles of the arm of the one who handles the instrument ; if so, 

 the extra weight has accomplished something." 



Ahrens' Erecting Microscope. — A point of considerable interest has 

 been found in connection with Ahrens' new erecting Microscope, figured 

 in this Journal for 1888, p. 1020, fig. 161. The erection is performed 

 by means of " Porro " prisms. In the description it is said that " the 

 erection of the image is obtained by two right-angled prisms crossed in 

 the way used in some of the binocular field-glasses." This is probably 

 the first application 1 of " Porro " prisms to the Microscope ; and the 

 passage quoted shows that the new field-glasses are not so new as some 

 of us believed. 



(2) Eye-pieces and Objectives. 



Modern Apo chromatic Objectives.* — Dr. H. van Heurck, after 

 some historical and theoretical introduction, summarises the chief forms 

 of modern apochromats. 



(1) Objective 16 mm., N.A. 0*30 (fig. 13). — The system consists of 

 three lenses : the frontal plano-convex of low curvature ; the median 

 double ; and the superior slightly convex, and in reality formed of a 

 highly curved biconvex lens between two menisci. The low numerical 

 aperture of this objective permits its employment only for histological 

 . studies, for which it gives very beautiful and delicate images. 



Fig. 13. 



FiG. 14. 



Fig. 15. 



(2) Objective 8 mm., N.A. • 65 (fig. 14). — This is a quadruple com- 

 bination of seven simple lenses of different kinds of glass, and of differ- 

 ent curves. It is a very successful objective, and if a microscopist 

 possesses only one apochromat he ought to select this. With different 

 compensating oculars the magnification varies from 62 to 562 diameters. 

 It shows Nobert's sixth group well with axial illumination, and resolves 

 the stria3 of Pleurosigma angulatum. 



(3) Objective 6 mm., N.A. 0-95 (fig. 15). — The construction "of this 

 objective resembles that of the last, but the curves are sharper, especially 



* Ann. Soc. Beige de Micr., xxiii. (1899) pp. 43-73 (1 pi. and 6 figs.). 



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