ZOOLOGY AND BOTANY, MICKOSOOPY, ETC. 317 



engraved on the setting (taken from the nose-piece to the eye-lens), 

 the objectives have their best correction for a cover-glass of 16-0 • 18 

 mm. Much thinner covers require a lengthening of the tube by 

 10-25 mm. further. They are very sensitive in regard to length of 

 tube, and the change in this length is the simplest, and in fact the 

 best, means for slight corrections for different covers — the reason 

 being that a change of that kind does not alter the proper balance of 

 the various corrections (spherical, chromatic and sphero-chromatic), 

 whilst an alteration in the distance of the lenses of the objective from 

 one another, as is done by a screw-collar, does disturb that balance 

 to the injury of the performance of the objective. It may be possible 

 to find a formula which will be less sensitive in regard to this 

 question of correction, but until it is found, Dr. Zeiss, by whom the 

 objectives are made, will not supply any with correction-collars, so 

 as to convert a good objective into a medium one for the sake of a 

 non-essential convenience only. 



A novel point in connection with the objective is that its 

 performance is improved by the use of special eye-pieces, of which 

 two are supplied, of 25 mm. and 15 mm. focal length. Their func- 

 tion is to compensate for certain aberrations outside the axis, which 

 cannot be compensated for in the objective. With these eye-pieces, 

 particularly with that of 25 mm. focal length, the field of view is 

 surprisingly uniform. 



Of the ten lenses of which the objective is composed, two only 

 are of siliceous glass, the other eight being made of borates and 

 phosphates. The crown and flint glass now used by opticians does 

 not contain (as essential components) more than six chemical elements, 

 O, Ca, K, Na, Pb and Si, whilst the new objective contains not less 

 than fourteen elements. 



The optical principle on which the objectives have been con- 

 structed is indicated in a paper by Prof. Abbe in this Journal,* " On 

 new methods for improving spherical correction," &c. In fact, all 

 the work of Prof. Abbe and Dr. Schott during the five years has 

 been solely directed to finding the proper means for the realization 

 of the desideratum there mentioned, viz. doing away with the 

 secondary chromatic aberration, and with the chromatic difference 

 of spherical aberration. The proper means was foimd in special 

 kinds of glass, which allowed of proportional dispersions in different 

 parts of the spectrum, and which at the same time exhibit different 

 relations between the refractive indices and dispersive powers. By these 

 means a more perfect concentration of all the rays emanating from 

 the object is obtained. With the old kinds of crown and flint glass 

 two different colours only could be collected to one focus, a secondary 

 spectrum remaining uncorrected, whilst the new objectives collect 

 three rays of different colours to one focus, leaving a small tertiary 

 spectrum only. Moreover, spherical correction has hitherto been con- 

 fined to rays of one colour, being made for the central part of the 

 spectrum, the objective remaining iinder-coirecied spherically for the 



» See this Journal, 11. (1879) p. 42. 



