618 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Such an objective is tlien somewhat over-corrected, and thus exactly- 

 suited for a Microscope objective, because in the case of a single lens 

 the over-correction can be removed by the Huyghenian ocular, while 

 with doublets and triplets, the lens can be corrected or over-corrected 

 to the desired amount, the residue being removed by the ocular as is 

 commonly done by the Lister method. 



When the necessary calculation for a given mean refractive and 

 dispersive relation, such as from quartz to a fluid, is once made for a 

 fixed large angle of aperture and a given thickness of the lens, it is 

 easily seen what alterations a change in the refraction of the less 

 refracting lens requires, according to the crown glass used, and we 

 can correct the objective accordingly. 



An objective, composed of three achromatics, whose curves were 

 calculated for parallel rays (according to the formulae and tables of 

 the author) gave such satisfactory results that further detailed 

 calculation is only required for exceptionally large angles of aperture. 



The performance of a triplet of 8 mm. equivalent focus composed 

 of three symmetrical endomersion lenses consisting of crown glass 

 and a mixture of fatty and aromatic substances, gave perfect achro- 

 matism, for when achromatic eye-pieces (by Schroder) were used 

 which magnified 9, 18, 36, and 72 times, there was even with the last, 

 in bright lamplight and sunlight, scarcely a trace of colour on 

 diatoms or on a Zeiss's silver grating, whilst all the objectives at 

 hand * showed all the colours of the spectrum with such enormous 

 eye-piece power. 



With some of these objectives, however, the aplanatism was more 

 perfect than others, which can probably be accounted for by slightly 

 imperfect centering of the three lenses, and also by the defective 

 quality of the plane-parallel plates, in place of which, later on, con- 

 cave lenses of great focal length were used. 



In direct light, with an angle of aperture of only 56°, all the 

 more easy diatoms of MoUer's plates were resolved, and of the more 

 difficult the following: — BJiahdonema arcuatum and It. adriaticum, 

 Achnanihes suhsessilis, Scoliopleura convexa (the images appear black 

 upon white). 



With oblique light : — Nitzschia circumsata, Navicula divergens, N. 

 minor, Gomphonema geminatum, Melosira Borrerii, SymholopJiora Trini- 

 tatis, Odontodiscus suhtilis, Hyalodiscus stelliger, and H. suhtilis could 

 not be quite resolved, as they were on the limits of the unresolvable 

 with the aperture. Grammatophora marina and Pleurosigma angu- 

 latum were not resolved. 



A double symmetrical endomersion objective, combined after the 

 manner of Steinheil's " Symmetric Aplanaten," gave no trace of a 

 difference of the chemical and visual foci, and therefore such an 

 objective, which can be constructed from quartz and a very transparent 

 fluid, is of practical importance for photography. 



The usual contrivance is not necessary for obtaining sharp photo- 

 graphs of diatoms, which will even bear well a power of 30 times as 



* Objectives by Schneider of Berlin, 1" to ^" dry, and ^-j\, (sic) immersion, 

 by Zeiss 1 n (A) and Hartnack, as well as Keichert of Vienna. 



