THE MICROSCOPE. 



viewed by the naked eyes, as well as in microscopes, especially 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 little time the effect wears off, and we again see it 

 in bas-relief ; then again in alt ; and so on, by successive fits. 

 This deception arises from the simple circumstance that the lights 

 and shades in bas-relief are very nearly like those of an alto- 

 relievo of the same subject, illuminated from the opposite side ; 

 our understanding in this case instantly corrects the false testi- 

 mony of the eye, when we consider from ivhich side the light 

 comes. (If we observe with a microscope, we must remember that 

 its image is inverted, and that in consequence the light must be 

 considered as proceeding from the side of the field of view oppo- 

 site to that w r here the source of illumination actually exists.) It 

 will also be highly advisable, when we are in doubt as to the 

 manner in which an instrument shows prominences and depres- 

 sions, to verify its vision by observing some known object with it, 

 of the real state of which, as to inequality of surface, we have 

 been previously informed by the sense of touch, to which it has 

 been well said there is no fellow." * 



* * * We usually see objects illuminated from above with the shadoivs beloiv 

 the prominences ; now, unless the light is below an opaque object, when 

 we view it in an engiscope, we shall see the shadoivs above, giving the 

 prominences the appearance of depressions, and producing a very unnatural 

 effect." 



48 



