W. LeConte Stevens — Physiologic! (),>ti, 



them extending their explorath 

 die land, ami into districts where there is most to be discovered. 

 All can help on the work, and all are doing so, by the communi- 

 cation of specimens and of observations. Those within the 

 range of the published manuals and floras get on — or should 

 get on— with only occasional help from us. 'They should send 

 us notes and specimens to any amount; but they should not 

 ask us to stop to examine and name their plants, except in 

 •s. which wo arc always ready enough to take up. 

 Those who collect in regions as yet destitute of such advan- 

 tages may claim more aid, and we take great pains to render it; 

 partly on our own account, that we maw assort their contribu- 

 tions into their proper places, partly for the encouragement of 

 such correspondents, who otherwise would not know what they 

 liave obtained, ami who naturally like to know when they have 



But the scattered and piecemeal study of plants is neither 



i ctoi-y nor sttfe. And it involves great loss of time, 

 b. sides interrupting that continuity and concentration of atten- 

 tion which the proper study of any group of plants demands. 

 As respects the orders of plants which are yet to be elaborated 

 for the Flora, and as to plants which require critical study or 



better to defer their complete determination until the groups to 

 which they severally belong are regularly taken in hand. 

 The cooperation of all our botanical associates is solicited in 

 • i, as a matter of common interest and advantage. 

 For we are ; progress of 



the Flora of North America : and we may confidently expect 

 from our botanical associates their sympathy, their forbearance, 

 and their continued aid. 



Akt. XXXVV- Sohsnn Physiological Optics. No. 6. 

 far Union oj Spn-tral. Images; by W. LeCoNT. 



If a sharply defined object be momentarily i 

 the intense light of the electric spark, a positn 

 negative after image is perceived, neither, howev 

 ] ong. A negai 

 secured by gazing veiy steadily on one point of a 



