104 . THE AMERICAN MOIS-THLY [June 



observational error. The results agreed with Lord Ray- 

 leigh's calculation and experiments, not with the Abbe 

 calculation or with Airy's. 



14. Dr. Stoney recognizes essentially what is here 

 maintained. " The standard image is the outcome? 

 partly of the features upon the object, and partly of the 

 state of the light by which the object is illuminated. It 

 may he improved by increasing the degree in which the 

 first of these factors^ and by decreasing the degree in 

 which the second, contributes to produce, to modify, or to 

 efface detail in the image." So closely does this practi- 

 cally coincide with my proposition, that had it stood 

 alone or as the final conclusion of his exposition, nothing 

 more would have been necessary ; and it has the further 

 merit of recognizing the fact (which constitutes the real 

 place and proportion of the " spectrum" theory in mi- 

 croscopy, and the nexus between it and the Airy theory) 

 that we have two distinct elements to deal with in an 

 image, whose respective preponderance or proportion are 

 highly variable. The present attempt at further treat- 

 ment is made chiefly because he does not seem to recog- 

 nize the true relative proportions, either in maintaining 

 with Abbe in such a universal sense that " diffracted 

 light is the machinery by which good definition is 

 brought about ; " or " the great assistance which is 

 rendered to the practical microscopist by Abbe's theory." 

 (To be Continued.) 



Woods.~L. W. Hahn, Silver Creek, N. Y., offers 110 

 varieties of foreign and native woods for $3.00. 



Personal. — Prof. W. A. Eogers died at Waterville, 

 Maine, March 1, 1898, aged 61 years. He had been pro- 

 fessor of physics and astronomy in Colby University since 

 1886 but expected to remove to Alfred University the 

 present spring. 



