ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



251 



The makers have full confidence in their objectives and guarantee them. 

 Figs. 50-52 show three of their achromats. Fig. 50 represents a power 

 about 2/3 ; it consists of a triplet front and a doublet back, each care- 

 fully corrected. Fig. 51 is a dry 1/6 power. A hemispherical front 

 lens is combined with a doublet middle and triplet back, the front 

 lens being the chief magnifier of the combination, and the other lenses 

 correcting the various aberrations. Fig. 52 is an oil-immersion ; it 

 consists of a hemispherical front lens, behind which is a meniscus, in 

 its turn followed by a doublet and a triplet ; these latter act as correct- 

 ing lenses. 



The apochromats are made of a different kind of glass, and of course 

 give a more perfect correction of chromatic aberration. This advantage 

 is not gained, without a certain sacrifice of simplicity in construction, 



Fig. 50. 



Fig. 51. 



Fig. 52. 



as a result of which the apochromat is more likely to suffer from 

 careless handling and from atmospheric changes than the more simple 

 achromat. 



As regards sharpness of definition and brightness of field, there is 

 little choice between the two classes of objectives of the same magni- 

 fication. The apochromats do, as a matter of fact, resolve the fine 

 markings of test objects somewhat more clearly than achromats, but the 

 difference is slight, and in ordinary stained microscopic preparations 

 is hardly detectable. 



The correction of both classes of objectives is complete. The 

 ordinary Huyghenian eye-pieces are consequently well adapted for use 

 with either. 



The achromats and Huyghenian eye-pieces are also well adapted^to 

 the requirements of photomicrography. 



One very important recommendation of the achromats is their re- 

 markably low cost. 



Leitz' Objectives for the Edinger Apparatus. — Figs. 53-55 show 

 the objectives of respectively 64, 42, and 24 mm. focal distance designed 

 for the Edinger projection apparatus. In manufacturing these objectives, 



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