1889.] MICROSCOPICAL JOURNAL. 57 



the device with minor points of difference from Prof. Abbe's and from 

 each other's. 



The condenser, as made by Mr. Zentmayer, is in two forms, one for 

 use with objectives having a numerical aperture of not more than 1.20, 

 the other for the widest angled glasses, itself having a numerical aper- 

 ture of 1.40. The purchaser should, before final selection, consider 

 which form he needs, his objectives deciding the question. 



It is not achromatic. As to the desirability of seeking achromatism 

 for it, microscopists differ. Mr. James Swift states that " the superi- 

 ority of light from an achromatic condenser over that of any non-achro- 

 matic arrangement is due to the fact that rays, in their passage through 

 a simple lens or combination of simple lenses, are decomposed into 

 their elementary colors, which seriously impair the beauty of uncolored 

 objects, such as the Pod?tra scale, etc., whereas, in the achromatic 

 condenser this defect is obviated, and all objects are seen with natural 

 colors ; moreover, confused pencils of light are produced by the spheri- 

 cal aberration of the single lenses which fogs the image of fine struc- 

 ture, whilst the achromatic condenser, being thoroughly corrected for 

 spherical aberration, provides illumination of the greatest purity, and 

 the most delicate objects are seen with a clearness and sharpness of de- 

 tail quite unknown to those microscopists whose experience has been 

 confined to the use of non-achromatic condensers." Prof. Abbe, how- 

 ever, says, " the condenser is not made achromatic for the reason that, 

 for the effect contemplated, it would be altogether useless to seek to ob- 

 tain a sharp image of the cloud or other source of light, as it is in like 

 manner quite immaterial whether the image is formed precisely on a 

 level with the object, or somewhat above or below it." Mr. E. M. 

 Nelson also takes issue with Prof. Abbe's opinion. But no optician 

 in America, so far as I am aware, has offered microscopists an achro- 

 matic wide-angled condenser. The only one made, I believe, is the 

 oil-immersion of Messrs. Powell and Lealand, of London. 



The modification of the Abbe condenser with 1.20 N. A. has but 

 two component lenses, a large bi-convex posterior one, with surfaces 

 of unequal curvature, and an anterior which is more than a hemisphere 

 in form. The other, with 1.40 numerical aperture, consists of three 

 lenses, the posterior being similar to that of the smaller angled form, 

 while the middle lens is concavo-convex, and the anterior a smaller 

 hemisphere. 



When used with wide-angled objectives, to avoid loss of light and to 

 obtain the best results, the space between the lower surface of the slide 

 and the top of the condenser should be filled with water or homogene- 

 ous immersion fluid, and in all cases, except when oblique illumina- 

 tion is desired by a lateral movement of the condenser, it must be ac- 

 curately centred. This is essential to its best performance. For ordi- 

 nary purposes, however, with dry objectives it may be used dry. 



Its focus is only a short distance below the object, or the upper sur- 

 face of the slide. This distance varies with the aperture of the special 

 form used, and is to be ascertained by experiment, but- 1 think the 

 condenser can seldom be employed when accurately focused, except, 

 perhaps, while studying the striae of diatoms with high power objectives 

 and high power eye-pieces. With medium powers the exact focus of 

 the condenser produces a little spot of light of terrible intensity. It is, , 



