792 Botanical Microtechnique 



N.A. greater than 1.25. One or both upper lenses are removable, 

 givino; N.A. 0.70 and 0.40 respectively. 



Ajjlanatic and achromatic condensers, made by several manu- 

 facturers, have excellent corrections lor color and curvature. The 

 elements, usually in 3 units, are separable, affording combinations 

 with N.A. ranging from 0.20 to the fidl 1.30 or 1.40 of the complete 

 condenser. 



Hie resolving power inherent in an objcctixe can be utilized only 

 if the illuminating system has a numerical aperture equal to that of 

 the objective. A curved mirror has an approximate N.A. of 0.25: 

 therefore, it meets the aperture requirements of a lOx (16 mm.) 

 objective. Microscopes having objectives of over N.A. 0.25 should 

 be cquipjjcd with a condenser, provided that the users are sufficiently 

 skilled to use the condenser properly. An improperly adjusted 

 condenser is worse than having no condenser. Some teachers prefer 

 not to have condensers for large elementary classes in which thorough 

 training in microscopy and close supervision are difficult. 



The conventional high-power objective on elementary class 

 microscopes is a 4-mm. objective, 43x or 44x. N.A. 0.65 or 0.66. 

 Many thousand instruments of this type are in use, ecjuippcd wiih an 

 Abbe condenser of N.A. 1.20 or 1.25. If this condenser is not focused 

 accurately it is a handicap, furthermore it does not co\er the field of 

 objectives below lOX- Removal of the condenser or of its upper 

 element, a common jjractice among advanced workers when using low 

 jiowers, is a most undesirable practice in large classes of bcginneis. 

 The need for a condenser designed specilicalh for low and inter- 

 mediate powers has been met by the American Optical C^o. (Spencer 

 Lens C^o.) and the Bausch 8: Lomb 0])tical C>o. Ihese condensers have 

 ninncrical apcriures of .(>() and .70 rcspcc ti\ely. and therefore meet 

 the aj)ci tuie rec|uirements of 4 mm.. 43x or 44x objectixes. and aKo 

 illunu'iiaic tiu' (icld of a 3.2x "•' higher power object i\e. 



A maximum N.A. of 1.00 can l)e obtained wiiU a (ondenser if the 

 condenser lens and the slide are separated 1)\ a hiNcr of air. Obviously, 

 an oil-immersion ol)je(ti\e of N.A. 1.30 does not \iekl maxinuim 

 j^erformance luiless the (ondenser, as well as the object ixc is (onnccicd 

 to the slide Avith (cdar oil. Rcsearcli woi kers Avho \\ish to obtain 

 maxinunn resohing power make a routine practice ol iiiuiici^ing the 

 condenser. There are some practical objections to this practice for 

 classwork. 



Dark-field illumination is a neglected, but usclul iniihod of 

 observation. In this nuiliod the light that reaches the eye from the 



