DEFINING AND PENETRATING POWER, 91 



1. DEFINING AND PENETRATING POWER ACCORDING TO THE 

 EARLIER AUTHORS. 



With reference to the optical power of the Microscope, micro- 

 scopists are accustomed to distinguish two different properties which 

 are manifest in the testing of the instrument, viz., the so-called 

 defining power, and the penetrating or differentiating power. By 

 the defining power of a Microscope is understood the capacity 

 of exhibiting clearly and distinctly in the image the form and 

 outlines of the object ; and by penetrating power, the capacity of 

 displaying fine structural details, such as layers of cell-membrane, 

 the markings of diatoms, &c. This distinction was first established 

 by Sir W. Herschel for telescopes, and was applied, later on, by 

 Goring to the Microscope. Herschel stated that telescopes with 

 rather large apertures, even if otherwise defectively constructed, 

 are particularly adapted to render visible obscure nebulie and con- 

 stellations, which cannot be distinguished with smaller instruments- 

 of the best kind ; on the other hand, the latter will show closely 

 adjacent bright points separated from each other, which would 

 appear in the former as if forming a single point. 



The penetrating power, or piercing power, is accordingly depen- 

 dent upon the size of the aperture ; the defining power, or outline 

 power, upon the accuracy of construction of the instrument, i.e.,. 

 upon the correction of the aberrations. It is also evident that every 

 optical instrument, as well as the naked eye, gives sharper images 

 the nearer the foci of the different-coloured and differently-inclined 

 rays on the retina ; and that, on the other hand, slight differences 

 in the luminous power of dim object-points are more apparent the 

 greater the quantity of light reaching the retina from such points. 

 Hence animals with large pupils see at night more distinctly than 

 man, and obscure objects are rendered more distinct to the human 

 eye in proportion to the enlargement of the pupil. 



The distinction between defining and penetrating power, which 

 Herschel established for the telescope, is therefore fully confirmed 

 for every optical apparatus, the naked eye not excepted; but it 

 must not be forgotten that the penetrating power varies propor- 

 tionally to the aperture of the pencil of rays issuing from the object 

 and reaching the eye, not with that of the refracting apparatus. It 



