LIMITS OF OPTICAL CAPACITY OF THE MICROSCOPE. 429 



observer can distinguish the minutest objects visible to him under 

 any circumstances. 



Calculated by the equation before mentioned, the diameter 

 (1"89 m.m.) of the area of light-rays entering the pupil, when the 

 light incident on the objective (in air) spreads out to nearly 180**, 

 corresponds to an amplification of 264j. For objectives with less 

 aperture the amplification must be set down at a lower figure. In 

 H. V. Mohl's handbook of the microscope it is stated, that 

 amplifications varying between 300 and 400 allow most detail to be 

 seen, whilst Harting, speaking of more recent instruments with 

 large angular aperture, found amplifications of 430 to 450 most 

 serviceable. 



If now it be required to determine the magnitude of the minutest 

 recognisable object as a standard by which to measure the accuracy 

 of the microscopic image, we must not take for our unit the 

 measured diameter of such objects as bright single spots or lines on 

 a dark field, or vice versa, for the reasons which I have already 

 given in my handbook of physiological optics (p. 217), in discussing 

 the capacity of the eye for distinct vision. Por, in the cases above 

 mentioned the result depends not only on the proportional 

 magnitudes of the images, but also on the susceptibility of the retina 

 to slight difi'erences of light. The most suitable objects are, here 

 also, fine gratings which shew alternate clear and dark stripes. 

 Such indeed are in common use, as in the examples of Robert's 

 lines, and the line-systems of diatoms and insect scales. But as the 

 light of the bright stripes is doubtless strongly dispersed before it 

 becomes quite undiscernable, dependence can be placed only on the 

 measurement of the space between the centres of two contiguous 

 stripes, and not upon the measurement of space occupied by the 

 stripes (wide or narrow) as originally distributed. I select, 

 therefore, as the measure of the minatest distinguishable objects^ 

 that smallest appreciable interspace between the centres of two 

 contiguous stripes by which these stripes can still be recognised 

 as separate. 



When diffraction is caused by a fine network of square meshes, 



