ZOOLOG? AND BOTANY, MICROSCOPY, ETC. 849 



To mark the place of any small object it is merely necessary to 

 read off the number on the millimeter scale, fig. 170. This gives what 

 may be termed the movable ordinate. The fixed ordinate is obtained 

 by keeping the line — ° on L' at a prearranged point, or by having 

 another millimeter scale marked along this side of the stage. Thus 

 any spot in a specimen can always be found at once. 



The object of the central widening of the frame t is for the admis- 

 sion of slides of the " Giessener Format." The longer length and 

 shorter breadth of t are for slides of the English and Vienna sizes. 



Zeiss's Apochromatic Objectives, Compensating Eye-pieces, and 

 Projection Eye-pieces. — As a rule we do not notice in the Summary 

 the catalogues or price lists of manufacturers, but the catalogue just 

 issued by Dr. C. Zeiss * of the new objectives and eye-pieces, contains 

 so much interesting as well as useful information for microscopists 

 that we reproduce it nearly in extenso. 



Apochromatic Objectives. — In the construction of the objectives thus 

 designated, new kinds of glass and a greatly improved method of cor- 

 rection have been employed, with the result that the secondary spec- 

 trum is removed, and the spherical aberration uniformly corrected for 

 the different parts of the spectrum. There is, therefore, a much more 

 perfect concentration of the rays in the image than with the best 

 objectives hitherto made, and in the case of the chemically effective 

 rays, there is neither focal difference nor spherical aberration. 



They also allow very high eye-pieces to be used without detriment 

 to the accuracy or brightness of the image, thus giving high magni- 

 fying power with relatively long focal length, and enabling a series of 

 very varying amplifications to be obtained with the same objective. 



The natural colours of objects, even in the more delicate tints, arc 

 reproduced unaltered by these objectives, in consequence of the very 

 slight intensity of the residual tertiary spectrum. 



The differences in the amplification of the image for the various 

 colours are reduced to the same amount in all the objectives, and are 

 removed by the compensating eye-pieces hereafter described. The 

 images therefore are uniformly free from colour throughout the 

 whole field of view. 



The spherical aberration outside the axis is so completely cor- 

 rected that the sharpness of outline existing in the centre of the field 

 of view is maintained almost up to the margin, although the focal 

 adjustment between the centre and margins is necessarily somewhat 

 different in consequence of the unavoidable curvature of the surface 

 of the image. 



The construction of each objective is based on calculations which 

 extend to the smallest details of optical action. Every element — radii 

 of curvature, thickness, diameter, and distance of the lenses from one 

 another — are all accurately adjusted and numerically determined 

 for each objective, with regard to the spectrometrical constants of 

 the various kinds of glass employed, and the numerous conditions 

 which have to be simultaneously fulfilled. The technical execution is 



* Zeis3, C, ' Xeue Mikroskop-Objective und Oculare aus Special-Glasern des 

 Glastechnischen Laboratoriums (Schott and Gen.),' 14 pp., 8v<>, Jena, 1886. 

 Ser. 2.— Vol. VI. 3 K 



