MICROSCOPIC VISION. 155 



twofold origin ; for whilst coarse structure was imaged 

 according to the ordinary dioptrical laws of light, the image 

 of the fine structure was dependent upon diffraction pheno- 

 mena, the investigation of which lay rather in the region of 

 physics than of geometrical optics. The line of demarcation 

 between coarse and fine structure was placed at y-Vo i^^^ j 

 therefore all objects larger than that size were dioptric or 

 absorption images, and all smaller ones were diffraction 

 images. This position is a wholly untenable one, for the 

 diffraction pencils arising from gratings, such as wire 

 sieves, linen threads, ruled scales, etc., where the intervals 

 are, at the least, -^\ inch, can be easily seen without 

 any special apparatus. With suitable apparatus the 

 spectra arising from much larger gratings have been made 

 visible. 



The two following are simple, though very important 

 experiments which prove that the Frauenhofer diffraction 

 law applies even to large objects which can be seen without 

 instrumental aid. 



1. When a scale on a carpenter's rule is examined through 

 a diaphragm, held close to the eye, the hole in it being 'Oil 

 inch in diameter, some divisions on the rule can just be 

 perceived at Ih inches ; what is the fineness of the 

 divisions ? 



Let a be the diameter of the hole, and X the wave length, 

 say ^^^00 inch, S being the value of one line and interspace. 



Then — will be the angle the hole subtends at the distance 

 7-5 * 



of the scale. By the Frauenhofer law u, the divergence of 

 the diffraction beams is such that sin w=«s, but in order 



that two diffracted beams may just pass through the hole 

 which is the condition of the limit of visibility of the 



