54 THE MICROSCOPIST. 



between such motions and the vital or voluntary motions 

 of organized bodies. 



O 



The inflection or diffraction of light is another source 

 of error, since the sharpness of outline in an object is thus 

 impaired. The shadow of an opaque object in a divergent 

 pencil of light presents, not sharp, well-defined edges, but 

 a gradual shading off, from which it is inferred that the 

 rays do not pass from the edge of the object in the same 

 line as they come to it. This is in consequence of the 

 undulatory nature of light. When any system of waves 

 meets with an obstacle, subsidiary systems of waves will 

 be formed round the edge of the obstacle and be propagated 

 simultaneously with the original undulations. For a cer- 

 tain space around the lines in* which the rays, grazing the 

 edge of the opaque body, would have proceeded, the two 

 systems of undulation will intersect and produce the phe- 

 nomena of interference. If the opaque body be very small, 

 and the distance from the luminous point proportionally 

 large, the two pencils formed by inflection will intersect, 

 and all the phenomena of interference will become evident. 

 Thus, if the light be homogeneous, a bright line of light 

 will be formed under the centre of the opaque object, out- 

 side of which will be dark lines, and then bright and dark 

 lines alternately. If the light be compound solar light, a 

 series of colored fringes will be formed. In addition to 

 the results of inflection, oblique illumination at certain 

 angles produces a double image, or a kind of overlying 

 shadow, sometimes called the ''diffraction spectrum," 

 although due to a different cause. No rules can be given 

 for avoiding errors from these optical appearances, but 

 practice will enable one to overcome them, as it were, 

 instinctively. 



Testing the Microscope. The defining power of an in- 

 strument depends on the correction of its spherical and 

 chromatic aberrations, and excellence may often be ob- 

 tained with objectives having but a moderate angle of 



