74 THE RESOLVING POWER OF MICROSCOPES 



width is one-third of the minimum interval for resolution. He cites 

 the following simple experiment : "In front of the naked eye was 

 held a piece of copper foil perforated by a fine needle hole. Ob- 

 served through this, the structure of some gauze just disappeared 

 at a distance from the eye equal to 17 inches, the gauze containing 

 46 meshes to the inch. On the other hand, a single wire .034 inches 

 in diameter remained fairly visible up to a distance of 20 feet or 

 240 inches. The ratio between the angles subtended by the periodic 

 structure of the gauze and the diameter of the wire was thus 



^^240 



.034 17 

 He finds for the proportionate loss of illumination at the centre of 

 the wire in this case 



■ '-'-0.11 



I'- 



about what might have been expected. 



The moral of these results is the recommendation of caution in 

 interpreting even the width of the bars causing the streaking in 

 microphotographs of pearlite, etc. 



Besides the references in the text, the following may be given. 



Airy, Tracts, 4th edit., p". 316 (reprinted as " Undulatory Theory 

 of Optics "); Astr. Monthly Notices, XXXIII., 1872; Camb. Phil. 

 Trans., 1834. 



Foucault, Ann. de I'Observ. de Paris, t.v., 1858. 



Verdet, Legons d'Optique, t.l, p. 265. 



Dawes, Mem. Astron. Soc, XXXV. 



Ch. Andre, Etude de la Diffraction dans les Instruments 

 d'Optique, Ann. de I'Ecole Norm., 1876. 



U. Behn, u. W. Heuse, Zur demonstrations der Abbeschen 

 Theorie des Mikroskops, Ber. d. deutsch Physik Ges. 4, 1906. 

 Physik Z. Schr. 7, 750, 1906. 



