324 THE SCIENCE OF MICROSCOPY. 



of the uncertainty of inference regarding structural conformation 

 (especially when drawn from images which represent the object 

 in piecemeal, i-^-, in successive optical planes) than Is afforded by 

 the irreconclleable differences of opinion exhibited in the various 

 **" resolutions " so confidently proclaimed by different observers. 

 To infer form from mere disposition of light and shadow in an 

 image which offers no direct suggestion of perspective is to court 

 error of observation. For when high ampllticatlons are employed 

 such light and shadow rather expresses differences of physical 

 aggregation, size, density, colour, nearness to each other, &c. of 

 the particles in any given optical plane than any continuous 

 structural outline of the whole object. And one of the strongest 

 illusions, that of surface elevation or depression suggested by 

 disposition of light and shadow, may be produced by a perfectly 

 plane surface, as Is proved by the use of artificial systems of lines 

 drawn on a silvered surface. Or, again, the particular appearances 

 accepted as ''resolution" of structure of this or that diatom may 

 be exactly counterfeited by lines drawn at given angles to each 

 other. The same kind of illusion occurs in ordinary vision as well 

 as in observing the microscope image, for instance, when viewing 

 figures (circles, squares, diamonds,) drawn In white on black 

 paper either in direct front view or obliquely with the naked eye, 

 we see a different configuration from that which we know it to be.* 



*The mental illusions in these instances occur with objects seen by 

 reflected light. A microscopic image view with, transmitted light may be 

 compared with a " transparency." The pretty pictures, (landscape or 

 portrait) seen when a porcelain slab is moulded so as to give an "absorption" 

 image, suggest perspective in landscape, or raised or depressed parts in 

 portraits (as in sculptured bas-relief) according as the thickness of the 

 several parts of the slab is varied. The light and shadow picture so 

 produced is conformable with the irregularities of the porcelain slab in 

 one sense only, namely, the dependence of form in the picture upon mass of 

 substance intercepting light. But in organic substances varying density of 

 different particles increases the contrast of light and shade, or occasions 

 additional effects by dispersion of Hght. And these effects of structural 

 differentiation in the object are not necessarily associated with corresponding 

 areas of formed outline. 



