EREOKS OF INTERPRETATION. 181 



warning against hasty conclusions drawn from a too cursory ex- 

 amination. If the history of almost any scientific investigation 

 were fully made known, it would generally appear, that the sta- 

 bility and completeness. of the conclusions finally arrived at, had 

 only been attained after many modifications, or even entire altera- 

 tions, of doctrine. And it is, therefore, of such great importance 

 to the correctness of our conclusions, as to he almost essential, 

 that they should not be finally formed and announced, until they 

 have been tested in every conceivable mode. It is due to Science, 

 that it should be burdened with as few false facts and false doc- 

 trines as possible. It is due to other truth-seekers, that they 

 should not be misled, to the great waste of their time and pains, 

 by our errors. And it is due to ourselves, that we should not 

 commit our reputation to the chance of impairment, by the pre- 

 mature formation and publication of conclusions, which may be 

 at once reversed by other observers better informed than our- 

 selves, or may be proved to be fallacious at some future time, 

 perhaps even by our own more extended and careful researches. 

 The suspension of the adjudgment, whenever there seems room for 

 doubt, is a lesson inculcated by all those Philosophers who have 

 gained the highest repute for practical wisdom; and it is one 

 which the Microscopist cannot too soon learn, or too constantly 

 practise. 



96. Besides these general warnings, however, certain special 

 cautions should be given to the young Microscopist, with regard 

 to errors into which he is liable to be led, by the misinterpreta- 

 tion of appearances peculiar to objects thus viewed, even when 

 the very best instruments are employed. Thus the sharpness 

 of the outline of any transparent object is impaired by a change 

 in the course of the rays that merely pass hy it, which is termed 

 Inflection or Diffraction. If any opaque object be held in the 

 course of a cone of rays diverging from a focus, the shadow 

 which it will form upon a screen held to receive it, will not 

 possess .a well-defined edge, but will have as its boundary a 

 shaded hand, gradually increasing in brightness from the part of 

 the screen on which the shadow is most intense, to that on which 

 the illumination is most complete. If the light be homogeneous 

 in its quality, the shaded band will possess no colors of its own ; 

 btit if the light be decomposable, like the ordinary solar beam, 

 the band will exhibit prismatic fringes.^ It is obvious that such 

 a diffraction must exist in the rays transmitted through the sub- 

 stance, as well as along the edges, of transparent objects; and 

 that it must interfere with the perfect distinctness, not merely 

 of their outlines, but of their images, the various markings of 

 which are shadows of portions- that afford obstacles, more or 



' This phenomena is explained on the Undulatory Theory of light, by the disturbance 

 which takes place in the onward propagation of waves, when subsidiary centres of 

 Undulation are developed by the impact of the principal undulations on obstacles in 

 their course ; the chromatic d'lspersion being due to the inequality in the lengths of the 

 undulations proper to the severally colored rays. 



