PAPERS GIVING RUSTS OF NORTH AMERICA. 175 



changes in the host amount to conspicuous alterations that attract 

 the casual observer, as in the case of " cedar apples," and all the 

 more so because the distortions are often accompanied by brilliant 

 coloration. 



For the study of these small objects Schweinitz was dependent 

 upon lenses of poor definition and no considerable magnification. 

 His chief instrument was undoubtedly the pretentious one now in 

 the possession of his grandson, the eminent oculist of Philadelphia, 

 Dr. Geo. de Schweinitz.^ This is still in almost or quite as good 

 condition as when purchased probably some time prior to 1817. It 

 was evidently one of the best instruments to be had at that period.. 

 As was pointed out in an early paper pertaining to the rusts, the 

 first published on the subject by the senior author,'* a magnification 

 of dry spores amounting to seventy-five diameters will give an ap- 

 pearance answering to the most detailed parts of Schweinitz's diag- 

 noses. It is considered by Shear & Stevens,^ who kindly loaned to 

 the writers during the preparation of this paper copies of their 

 manuscripts embodying results of researches pertaining to Schwei- 

 nitz's scientific labors and collections, that Schweinitz had to deal 

 with a greater handicap than low magnification in his microscopic 

 work. They find that the lack of spherical and chromatic correc- 

 tion of the lenses and the poor illumination must have resulted in 

 decidedly inferior definition. 



But in many cases it is clear that Schweinitz drew up the descrip- 

 tions of his new species without making use of this instrument. 

 He doubtless had some form of hand lens, although considerable 

 inquiry has failed to reveal any present trace of such a glass. Even 

 a simple hand lens seems not to have been used at times, and in 

 general much dependence was placed upon the gross appearance 

 and the changes wrought in the host. 



It would be interesting to know what facilities in the way of 

 books were possessed by Schweinitz. Probably his botanical 



3 The instrument was kindly loaned by Dr. de Schweinitz for display 

 before the Society at the presentation of this paper, and is illustrated by 

 Shear and Stevens in Mycologia for July, 1917. 



* Arthur, "The Interpretation of Schweinitzian and Other Early Descrip- 

 tions," Amer. Nat.. 17: 77-78, Jan., 1883. 



^ Mycologia, g: 195, 1917. 



