500 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



nor an apochromat, but something between the two. They might, there- 

 fore, be appropriately called | apochromats. These glasses are of very 

 high quality, and their price is but little in advance of that of the 

 makers' ordinary lenses. 



Leitz' New Objectives.* — The Wetzlar firm has now produced 

 achromatic fluorite objectives, numbered 6a and 7a respectively. The 

 colour correction is more perfect than in Nos. 6 and 7, but the magnifi- 

 cation and numerical quantities are unaltered. The details are— 



The Notation of Microscopical Objectives.f — L. Malassez inquires 

 whether it may not be possible to evolve a uniform system of notation 

 applicable to all objectives. He points out how various and defective 

 all existing methods are, and expresses the opinion that objectives should 

 evidently be designated by some indication of their magnifying power. 

 The differential character of objectives depends chiefly upon the range 

 along the principal axis of the ultimate position of their characteristic 

 (i.e. the line forming the limit of all the magnifications which the objec- 

 tive is capable of producing). The more remote this ultimate position 

 the greater the magnification produced. To a smaller extent the differ- 

 ential character depends also upon the position of posterior focus of the 

 objectives ; the more remote this focus, the greater the magnification. 

 The objective notation should then be based upon these qualities. As 

 regards the distance of characteristic, the author proposes to represent it 

 by what he calls the specific magnification, viz. that produced by the 

 objective at each increasing unit of distance, or, in other words, that 

 which it produces at unit distance from its posterior focus. The deci- 

 meter should be taken as the unit of distance. This specific magnifica- 

 tion y can be evaluated in various ways : it may be obtained by merely 

 using micrometric oculars and taking any two magnifications whatever 

 (G, g), and noting the distance 8 between them ; it can be shown that 



G-g 



Among; other methods the author recommends the use of 



y = 



the Weiss focimeter. As regards the position of the posterior focus, 

 the author proposes the epithet posterior foco-facial for the distance 

 between this focus and the posterior (or issuing) face of the objective. 

 Moreover, as this posterior focus is sometimes behind this face (weak 

 objectives), sometimes in front of it (strong objectives), he employs the 

 letters p (post) and a (ante) to express the two cases respectively. This 

 distance, <f> p or </>„, can be easily calculated if one knows the specific 

 magnification y of the objective, any magnification produced by it, 

 and the distance d between the position of magnification and the pos- 



* Catalogue No. 41 (Mikroskope) 1905, p. 14. 



t Arch. Anat. Micr., vii., fuse. ii. pp. 270-350 (8 figs.). 



