OBJECTIVES AND OCULARS 9 



little importance, since only low powers are employed and the 

 indices of refraction of objects and surrounding medium are 

 generally sufficiently different to permit an easy study of the 

 preparations. When magnifications of from 300 to 500 are 

 required in microchemical examinations, difficulty will be expe- 

 rienced in obtaining suitable objectives unless the prospective 

 purchaser stipulates long working distances, since the working 

 distance of those manufactured for the use of biologists is far 

 too short to permit their application to the study of uncovered 

 and therefore thick drops of liquid. 



For the study of objects lying in a single plane, for polished 

 surfaces, rulings, fine etchings, etc., in which sharpness of out- 

 line and delicacy of structure or tracery are present, flatness of 

 field and high numerical aperture are essential. Our choice is, 

 consequently, here restricted to aplanatics or to apochromatics, 

 bearing in mind the fact that the resolving power of an immersion 

 objective, where applicable, is greater than that of a dry one. 



If, on the other hand, the investigation to be conducted 

 involves much photomicrographic work, photo-objectives, apo- 

 chromatics, or better still, the very carefully constructed micro- 

 planars, microsummars, or microanastigmats, should be selected. 

 For in addition to the fact that the chemical or actinic rays 

 are not properly brought to a focus, it should be remembered 

 that ordinary microscopic objectives are corrected for a fixed 

 tube length, usually 160 millimeters, while in the case of photo- 

 graphic work the distance between objective and plate holder 

 is variable and in all cases much greater than the standard tube 

 length. 



THE CARE OF OBJECTIVES. 



Objectives should always be most carefully handled and pro- 

 tected from dust and vapors. They should be kept dry and 

 clean by wiping with clean new lens paper. 1 Never use a 

 piece of lens paper more than once, nor touch the lenses of 

 objectives or oculars with the fingers or with cloths. 



1 "Lens paper" is a soft absorbent tissue-like paper made from long flexible 

 fibers expressly for cleaning lenses. 



