242 ox THE THEOEY OF THE MICKOSCOPE. 



striation exist at all; whether the visible delineation is caused 

 by isolated prominences, or depressions, &c., no microscope 

 however perfect, no amplification however magnified can inform 

 us. All that can be maintained is the mere presence of conditions 

 optically necessary for the diffraction effect which accompanies 

 the image-forming process. So iar, however, as this effect 

 is visible in any microscope (six symmetrically disposed 

 spectra inclined at about 65"^ to the direction of the undiffracted 

 rays, ordinary direct illumination being employed) it may 

 proceed from any structure which contains in its substance, or on 

 its surface, optically homogeneous elements arranged with some 

 approach to a system of equilateral triangles of 0*48^ dimensions 

 (=circa 3-^io o inch.) Whatever such elements maybe— organised 

 particles or mere differences of molecular aggregation (centres of 

 condensed matter) they will always present a delineation of the 

 familiar form. All ground for assuming these elements to be 

 depressions or prominences, fails, after proof that neither the 

 visibility of the markings, nor their greater distinctions under 

 oblique illumination, has anything to do with shadow effects.*" 

 The distribution of light and shade on the surface of the valve in 

 the form of a system of hexagonal fields, is the mathematically 

 necessary result of the interference of the seven isolated pencils of 

 light which is caused by diffraction, whatever may be the 

 physical condition of the object causing this diffraction: the 

 position of the hexagonal fields, with two sides parallel to the 

 middle ribs, has its sufficient reason in the visible disposition of 

 the diffracted spectra towards the axis of this valve, and can be 



* The cliang-es which the image of Pleurosigma angm. undergo when the 

 microscope is raised from or lowered upon the object, prove nothing in 

 respect to the existence of elevations on its surface, for the same changes 

 occur in the same way when diamond ruled lines on glass are examined. 

 And besides, when a sharply defined light is viewed through a Pleurosigma 

 valve, according to the method already described, no divergence from the 

 refraction of the rays passing through it can be recognised— the valve 

 behaves in this respect just as if it were a glass plate with parallel surfaces. 



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