THE MICHOSCOPE. 



heave that visual angle, it would follow that the smallest ohject 

 of which the form could be rendered distinctly visible by a 

 microscope of a given magnifying power, would be one whose 

 linear dimensions are as many times less than the 360th part of 

 an inch as there are units in the number expressing the magnify- 

 ing power. 



It must not be forgotten, however, in considering such points, 

 that the smallest object whose form can be distinctly seen at a 

 given distance without a glass, depends on many conditions, some 

 connected with the object, and some with the observer, as has been 

 already stated. 



Many persons fall into the error of supposing that the excel- 

 lence of a microscope is to be determined by the greatness of its 

 magnifying power. On the contrary, that instrument must be 

 considered the most efficient which renders the details of an object 

 perceptible with the lowest power. Distinctness of definition, 

 by which is meant, the power of rendering all the minute linea- 

 ments clearly seen, is a quality of greater importance than mere 

 magnifying power. Indeed, without this quality, mere magnify- 

 ing power ceases to have any value, since the object would appear 

 merely as a huge misty silhouette. 



Sufficiency of illumination is another condition which it is 

 difficult to combine with great magnifying power, but which is 

 absolutely necessary for distinct vision. 



If two instruments show the same object with equal distinct- 

 ness of definition and with sufficiency of illumination, one 

 having a higher magnifying power than the other, then it must 

 be admitted that the one which bears, with such conditions, the 

 higher power is the more efficient instrument. 



The mere magnifying power depends on the focal length of 

 the lenses, the illumination on the angle of aperture, and the 

 distinctness of definition on the extent to which those condi- 

 tions have been fulfilled which confer upon the combination of 

 lenses composing the instrument, the qualities of aplanatism and 

 achromatism. 



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