Objectives. 41 



running any serious risks of making mistakes in what he 

 sees.' 



This opinion is borne out by many of the most careful 

 and practical observers in the microscopical world ; still, as 

 before remarked, if original research is to be conducted, 

 and the very finest results are to be obtained, and the 

 minutest structure detected in the plainest manner, the 

 apochromatics should be selected. There is another advan- 

 tage which these lenses are claimed to possess : they will 

 work with very high-power eyepieces, so that a great variety 

 of magnification may be obtained with one objective. Pre- 

 suming an apochromatic objective of J-inch power, having an 

 initial magnification of 40 diameters, will stand an eyepiece 

 power of 27, a maximum magnification of 1,080 diameters, 

 and by means of the searcher ocular power 2, as low a 

 power as 80 diameters can be obtained with that one lens. 

 Now, this is a very wide range, and saves a lot of trouble 

 in unscrewing objectives to insert those of a higher power. 

 The best quality achromatic objectives also stand a fairly 

 high eyepiece power, say up to 16, or even 20, but not in 

 the perfect manner that the apochromatics do. Magnifying 

 power, however, is not the primary thing in connection with 

 an objective, unless there is a corresponding delineating 

 power. This latter quality is dependent on the aperture. 



Apertures of Objectives Angular and Numerical. 



On the aperture possessed by an objective depends the 

 amount of detail that can be discerned, and it is a most 

 important point in the selection of an objective. The 

 theory of microscopic vision, as formulated by Professor 

 Abbe, is well worth comprehending, and is continued in 

 extenso in the larger works on the microscope. It hardly 

 comes within the compass of this volume, which is in- 

 tended to be practical only. 



Until the introduction of immersion objectives, the 

 angular aperture of an objective was always given that is, 



