Tlie iiiaiuifactiire of all our objectives is based upon a method of coiuidete 
theoretical computation of all the constructive tyi)es ])revious to their actual manu- 
facture. By this process, which was introduced by Prof. Abbe in our works 
in 1<S(3<S and has since been adhered to by us, it has become possible to employ, 
through judicious division of labour constantly controlled by scientists, a large 
number of persons in the i)roduction of articles of the greatest possible perfection. 
Strictly mathematical computation of every detail of construction combined 
with exact technical methods of working and a systematic control of each phase 
of manufacture, obviate all emi)ii-ical tests and ensure extraordinary uniformity 
of our lenses from the highest to the lowest i)owers, at the same time altogether 
excluding specimens of inferior (juality. All objectives are uniformly free from 
spherical aberration up to the marginal zone (proper thickness of cover with 
the higher powers being understood) and, as far as possible, perfectly corrected 
for colour. Si)ecial consideration is also given to the removal of aberrations in 
the extra-axial i)art of the field and to flatness of field. 
Working- Distance. Owing to the imi)0]'tance of a good working distance 
for the convenient and safe employment of the higher i)owers. })articular attention 
has been given to this factor in calculating the formulae of the various glasses. 
Our higher objectives ])0ssess therefore unusually lai-gc working 
distances, in comparison with their focal length and aperture. 
Tube Length. All the objectives named in this catalogue are adjusted 
for a tube length of lOO mm ((P/a in.). excei)t where a remark to the contrary is 
exjjressly made in the following pages or unless an objective for another tube 
length be si)ecially ordered. The length is reckoned from the contact surface of 
the objective thread to the u])per end of the body on which the eye-piece rests. 
This may be read off directly on stands of our make by the divisions on the 
draw tube. 
T h e 0 b j e c t i v e s a, aa, A, 1> a n d C may be used on t h e 1 a r g e 
English stands with lO-incli tubes without ai)preciable loss. 
