100 THE USE OF THE MICROSCOPE 



pn'.pariilioiLs, if p()88il)lc, dry or in water, as well as in a 

 medium homogeneous with ghiss. 



Limit of Resolution. — An objective of a given aperture, 

 with a given cone of Hght from the condenser, has a limit 

 of separating (resolving) power, for markings, edges or 

 particles, at a certain distance apart. Below this limit 

 of separation the markings appear to run into one. This 

 limit of resolution has been calculated for a full cone of 

 perfectly focused light of a definite color, when the working 

 aperture is equal to the actual aperture of the objective; 

 which may be nearly (if not quite) the case with high 

 powers correctly adjusted. This Hmit is the distance at 

 w^hich two neighboring points are just distinct, and do not 

 appear as one. All particles smaller than this appear 

 spread out to about this size; and if black they look grey; 

 the lighter, the smaller they are; till below a certain size, 

 even black particles are indistinguishable from the bright 

 field. The limit of resolution of a grating of lines is theo- 

 retically given as 0.21 micron for vision, and 0.15 micron 

 for photography, for a perfect objective of 1.3 aperture, 

 with a full cone of light, with wave length of 0.55 micron 

 for sight, and 0.40 micron for the camera. According to 

 MerUn (96), an ordinary achromatic objective of 1.3 

 aperture, properly illuminated, can detect (submicroscopic) 

 particles or filaments 0.08 micron across; though their 

 true sizes and their details would of course not be visible. 



Limit of Useful Magnification. — In practice, the cal- 

 culated limit of resolution is important chiefly as leading 

 to an upper limit of useful magnification. This is usually 

 between about 530 and 1,060 times the working aperture, 

 varying with the acuity of vision of the observer, and 

 presupposing correct adjustment and critical microscopy. 

 It is usually taken as 1,000 times the working aperture. 

 The lowest limit of magnification may be taken as 250 

 times the objective aperture, where the outer eyepiece 

 circle is about equal to the pupil of the eye. But this 

 is too low, the eyepiece required being usually less than 5- 

 times. With accurate microscopy, 1 ,000 times the working 



