26 PKACTICAL PHOTO-MICKOGKAPHY 



The term ' resolving power ' means the power of resolving 

 or properly delineating fine regular structure, and must not be 

 confused with magnifying power or any arrangement by which 

 mere amplification is obtained. Neither does it indicate the 

 limit of visibility ; small isolated objects, or the components 

 of irregular structure, may be seen that are considerably smaller 

 than the distance between the components of a regular structure 

 such, for instance, as occurs in a diatom or a mechanically 

 ruled grating. Consequently it is desirable to use lenses of the 

 highest obtainable aperture, but of the longest focus in other 

 words, the lowest power lens that will show what is required. 

 It is seldom necessary to master the various methods of 

 measuring the N.A. of objectives, as this is usually marked on 

 the lens mount. Good makers now take care that the aperture 

 which they claim is justified ; but as it may happen that such 

 methods of measurement are required, the best known and 

 most reliable of these are described in the Appendix. A very 

 complete table is published with the Journal of the Eoyal 

 Microscopical Society, setting out the number of lines per inch 

 that may be resolved with a given aperture, under given 

 conditions of illumination, and reference should be made to 

 this if such information is required. 



It may at once be stated, that where the question of cost is 

 of no consequence, then a series of Zeiss apochromatic lenses 

 will place the photo-micrographer in the possession of as fine a 

 series of objectives, with the highest N.A., as can at present be 

 obtained. On the other hand, achromatic objectives are now 

 being improved so greatly and continually that no one need 

 despair of producing work of a very high class by their aid, 

 especially if care is taken, by methods to be explained later, to 

 ensure that any outstanding colour for which they are not 

 corrected is eliminated. 



Objectives are described in terms that represent their ap- 

 proximate initial magnifying power, without an eye-piece, and 

 this description does not of necessity indicate the distance which 

 there may be between the front of the lens and the object ; 

 although there is, speaking broadly, some relation between this 

 distance and the power of the objective. 



Owing to the desire for higher aperture lenses, the indication 

 by makers of focal length has become somewhat loose ; for it is 



