MICROSCOPIC ILLUSTRATIONS. 261 



hlance of depression. As almost all diaphanous bodies 

 can be examined as opaque objects, a scrutiny of them in 

 this way will generally be found greatly to assist our judg- 

 ment concerning their nature, whether they admit of being 

 cut into sections or not. It would be easy to write a 

 volume on this subject only, if we commenced an illustra- 

 tion of particulars which could not be rendered clear and 

 satisfactory without a vast number of figures. Long prac- 

 tice must after all determine our opinions, and scepticism 

 should ever form a leading feature in them; we should 

 suspect rather than believe. 



18. Opaque objects are not upon the whole so liable 

 to produce optical deceptions as transparent ones, because 

 we are more in the habit of viewing ordinary bodies by 

 reflected or radiated light. The most common illusion 

 presented by them is that of shewing a basso-relievo as an 

 alto-relievo; the reverse deception sometimes occur also, 

 but more rarely. This effect occurs in ordinary objects 

 viewed by the naked eyes, as well as in microscopes, espe- 

 cially if but one eye is employed.* Thus, if we look 

 intently for some time at a basso-relievo (a die of a coin, 

 for example), illuminated with very oblique light, it at first 

 appears in its true character ; but, after a little while, some 

 point on which we more particularly direct our gaze will 

 begin to appear in alt, the whole rapidly follows ; in a 



* The angle formed by the convergence of the two axes of our eyes directed 

 to any particular object, the distance from eye to eye being the base of the 

 triangle, enables us to judge pretty accurately of the distance of near objects ; 

 but, if only one eye is used, of course our measure of distance is gone. Now, 

 as the distance of a depression below or beyond a given plane must always be 

 greater than that of an equal elevation above or within it (cseteris paribus), 

 we are naturally less likely to be deceived when we use both eyes than when 

 one is rendered inactive, as must be the case in all optical instruments not of 

 a binocular construction. 



