= 8 = 



extraordinary degree of uniformity in the highest as well as in the lowest 

 powers of our lenses, whilst excluding the bare possibility of inferior 

 productions. All objectives are uniformly free from spherical aberrations up 

 to the marginal zone (assuming correct thickness of the cover-glass with the 

 higher powers) and, as far as possible, from chromatic aberrations also. Special 

 consideration is also given to the compensation of the aberrations in the extra- 

 axial part of the field and to the flatness of the image. 



Free working distance. Owing to the importance of a good working 

 distance for the convenient and safe employment of the higher powers particular 

 attention has been given to this factor in calculating the formulae of the various 

 glasses. Our high power objectives possess therefore unusually large work- 

 ing distances in comparison to their focal length and aperture (see Tables on 

 pp. 15, 22 and 26). 



Tube-length. The objectives specified in this catalogue, so long as 

 they are supplied in the ordinary mount, are all adjusted for a 

 tube-length of 160 mm (about 6y 2 in.), reckoned from the shoulder 

 of the objective screw to the upper end of the draw -tube on which 

 the ocular rests. In Fig. 1, this length is indicated by T. 



The tube-length may be read off on the larger stands of our make by the divisions 

 on the draw-tube. The interposition of any apparatus, such as a revolving nose-piece, a 

 sliding objective-changer, &c., between the tube and the objective necessitates the shorten- 

 ing of the tube by the length of such intermediate piece so as to make the whole distance 

 between objective shoulder and ocular equal to 16O mm (or about 6 1 /, inches). 



The position of the ocular is usually somewhat higher in the cane of var - 

 ous oculars of special construction, such as micrometer, screw micrometer, sjiec- 

 troscoiiic oculars, <c. When using such oculars this must therefore be ttttoived 

 for, objective-changers, &c,, being, if necessary, omitted in order to maintain the 

 correct tube-length. 



The lower-power achromatic objectives as far as C may also 

 without appreciable loss, be used on English stands with 10-inch 

 tubes. 



All the other objectives, particulary those of the apochro- 

 matic series, perform more or less imperfectly on stands of the 

 English type, unless specially adjusted. 



If the objectives are to be used with tubes differing from the 

 usual length, this fact should be expressly noted in the orders. 



Thickness of cover-glass. All objectives in fixed mounts are, unless other- 

 wise ordered, corrected for a medium thickness of cover-glass, between 0.15 and 

 0.20 mm. 



In the higher powers (from the apochromatic objective 8 mm and the 

 achromatic D upwards), with the exception of the homogeneous immersion ob- 



