Studies of the Schweinitz Collections of Fungi 195 



Although carried on as an avocation in the midst of pressing 

 church duties, his mycological work bears every evidence of care 

 and accuracy. In the studies on which were based the manu- 

 script taken to Europe in 1817 he certainly made use of a com- 

 pound microscope of considerable power. That this microscope 

 is the same one to which he refers in 1820 as the " great micro- 

 scope " 3 and which is now in the possession of his grandson, Dr. 

 Geo. de Schweinitz of Philadelphia (see Plates 8 and 9), is very 

 probable, since this instrument was manufactured by Adams of 

 London, and is of a type very closely resembling the " Jones Im- 

 proved Compound Microscope" (1) manufactured about 1798. 

 This microscope has a set of seven simple objectives numbered 1 

 to 7, as well as four lenses with polished metallic rims " specula " 

 and is in every respect one of the best instruments made at that 

 time. 



Examination and use of Schweinitz's microscope fully confirms 

 Arthur's (3) belief that the reasons for the errors in Schweinitz's 

 descriptions of microscopic characters are to be sought in the im- 

 perfections of his instrument. The chief causes of the mistakes 

 were, however, not so much the manner of mounting the spores 

 or the low magnification, as suggested by Arthur, but the lack of 

 spherical and chromatic correction of the lenses and the poor 

 illumination, resulting in very poor definition. An excellent 

 illustration of the imperfections of this instrument can be obtained 

 by examining spores of Schweinitz's Clasterisporium caricinum 

 with a modern microscope and comparing the appearance with his 

 drawings (13, Plate XIX, Fig. 4c), which are faithful representa- 

 tions of the spores as they appear under his microscope, using 

 objectives 1 and 2. 



This inevitable difference in the instruments used by mycologists 

 of successive generations, adds emphasis to the importance of 

 securing and ^ preserving authentic type specimens, and to the 

 value of such type specimens as compared with even the most 

 careful published descriptions. It emphasizes also, as suggested 

 by Arthur, the importance of noting very carefully the kind of 

 microscope used and especially the desirability of preserving 

 microscopes of every period in museums and laboratories. 



3 Letter to Torrey dated June 24, 1820. 



