448 ON APEETUEE AND DEFINITION OF MICEOSCOPE OBJECT GLASS, 



amount and quality of light admitted by larger aperture, 

 and they ignore a circumstance which really is, and should 

 be considered, the master condition of the problem, namely, 

 accompanying increase of magnifying power, because angular 

 aperture is a necessary accompaniment of the construction 

 by which increase of magnifying power is obtained (increase 

 of curvature and shortening of focal length of the lens). 

 I^ow the definition of the more magnified image may or may not be 

 improved, but if only not deteriorated the additional amplification 

 cannot but further separate and thus render visible (resolve) lines 

 that were too close to be seen separately with a glass of smaller 

 aperture and lower power. 



The fractions in the table indicate that separation of detail pro- 

 gresses in a marked manner with increase of angle up to 100° or 

 110°. And this range of increased aperture corresponds with the 

 deepening curves of construction of a lens by which its magnify- 

 ing power is raised. But, beyond the amplification suited to an 

 aperture of 110° (in an objective made to be used ''dry"), the 

 resolution of lines separated by clear interspaces is obtained with 

 greater difficulty, because the additional magnifying power spreads 

 the light over a larger image, whilst that image must be viewed 

 through a smaller optical aperture. As soon as light and shade are 

 less contrasted, sharpness of vision fails. And, with high power 

 lenses, difi'raction efi'ects are unavoidable if the illumination be 

 intense. Thus the limits of physiological vision are being 

 approached in two ways — first, by faintness of marking if the 

 brightness of image fails ; and secondly, by indistinctness of out- 

 line from overlapping images or blurring by interference waves. 

 The faculty of visual analysis is weakened or destroyed before the 

 theoretical limit (half the wave length of violet rays) is reached. 



ISTow, it is known that definition is improved in all low and 

 moderate amplifications by use of diaphragm openings, which 

 regulate and reduce the angular spread of the incident defining 

 pencils of light. But this narrowing of illuminating pencils 

 necessarily favours diffraction, whilst wide pencils (i.e., larger 



