1886.] 



MICROSCOPICAL JOURNAL. 



89 



gard 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 con'ections (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 in- 

 jury of the performance of the ob- 

 jective. It may be possible to find a 

 formula which will be less sensitive 

 in regard to this question of correc- 

 tion, 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 ob- 

 jective 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 sup- 

 plied, of 35 mm. and 15 mm. focal 

 length. Their function is to com- 

 pensate for certain aberrations outside 

 the axis, which cannot be compen- 

 sated for in the objective. With 

 these eye-pieces, particularly with 

 that of 35 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 be- 

 ing made of borates and phosphates. 

 The crown and flint glass now used 

 by opticians does not contain (as es- 

 sential components) moi'e than six 

 chemical elements, O, Ca, K, Na, 

 Pb, and Si, whilst the new objective 

 contains not less than fourteen ele- 

 ments. 



' The optical principle on which 

 the objectives have been constructed 

 is indicated in a paper by Prof. 

 Ab])e,* *■' On new methods for im- 

 proving spherical correction," &c. 

 In fact, all the work of Prof. Abbe 



"Journ. R. Micr. Soc. ii, (1879) 4^- 



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 secondaiy 

 chromatic aberration, and with the 

 chi'omatic difierence of spherical ab- 

 erration. The proper means was 

 found in special kinds of glass, 

 which allowed of proportional dis- 

 persions in different parts of the 

 spectrum, and which at the same 

 time exhibit different relations be- 

 tween the refractive indices and dis- 

 persive 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 

 colors only could be collected to one 

 focus, a secondary spectrum remain- 

 ing uncorrected, whilst the new ob- 

 jectives collect three rays of differ- 

 ent colors to one focus, leaving a 

 small tertiary spectrum only. More- 

 over, spherical correction has hith- 

 erto been confined to rays of one 

 color, being made for the central 

 part of the spectrum, the objective 

 remaining under-corrected spher- 

 ically for the red rays and oz'^^r-cor- 

 rected for the blue rays. In the new 

 objectives, however, the correction 

 of the spherical aberration is ob- 

 tained for Hvo different rays of the 

 spectrum, that is, practically for all 

 colors at the same time, and the ob- 

 jective shows the same degree of chro- 

 matic correction for the central as for 

 the marginal part of the aperture. All 

 this requires greater complication in 

 the construction, hence the use of 

 five lenses instead of the four hith- 

 erto employed. In addition, uni- 

 formity of amplification by the va- 

 rious zones of the clear aperture has 

 been obtained in a higher degree 

 than could hitlierto be done. 



' The objectives will be specially 

 useful in photo-micrography where 

 the correction of the secondary spec- 

 trum will be found of considerable 

 practical advantage. Not only is 

 there no difference in the optical 



