&6 THE AMERICAN MONTHLY [June 



and surely to ground such a physical postulate upon the 

 very imperfection of an imperfect tool, is rather arguing 

 in a circle. It seems to be a case of what was described 

 only the other day in a review of a mathematical work, 

 as *' the special philosophical vice of the mathematicians, 

 the tendency, namely, to mistake the sign for the thing 

 signified." 



7. This seems further to appear, when we consider 

 the reversal of the supposed dynamical system. This, it 

 is supposed, produces the "best attainable image which 

 the light emitted by the object [and grasped by the ob- 

 jective] is capable of producing." Unquestionably the 

 light-waves emitted may truly be regarded as a dynami- 

 cal system ; and may be conceived as reversed; and the 

 reversal of the whole actual system would produce such 

 an image as described. But it does not seem to follow 

 that mere " coalescence and interference of uniform plane 

 waves'' involves such a result. Besides what has 

 already been said as to the absence of plane-wave char- 

 acter in rays from any self-luminous object, at the very 

 minute focal distance of a high-power objective, questions 

 as to the longitudinal components in the disturbances, 

 and their disposal and influence, and several other ques- 

 tions, would seem to need further solution than is known 

 at present, before this could be assumed. 



In any case, what the reversal of the supposed dy- 

 namical system must really reproduce as an image at the 

 place of its origin, must be the postulated operative cause 

 of the system. That, by the hypothesis, is not an actual 

 object and it alone, emitting luminous waves, but the 

 object surrounded hy an indefinite number of identical 

 replicas, emitting identically similar plane waves. This 

 does not represent any object in reality : and that fact 

 seems to dispose of such a presentment as a full and com- 

 plete representation of microscopic vision. 



