April 26, 1895.] 



SCIENCE. 



471 



field could be erected than to arrange for 

 completing and caring for the work he 

 loved, and to which he gave freely so many 

 yeai*s of his life — namely, the Herbarium 

 of the Academy of Natural Sciences. 

 Mainly through his disinterested labors, it 

 stands to-day scarcely second to any in the 

 United States, containing, besides many 

 unnamed, over 35,000 named species of 

 flowering plants and ferns, the half of 

 which have been verified and fastened 

 down. 



" No one can probablj' be found to give 

 the years of time he so freely gave. In 

 order to carry on the work, and add to the 

 collection, as exploring expeditions afibrd 

 the opportunity, it has been proposed to 

 establish a Redfield Memorial Herbarium 

 Fund. 



" Mr. RedfieUFs will provides that his 

 herbarium, minerals, shells and scientific 

 works shall be sold to help the herbarium, 

 thus furnishing a nucleus for the proposed 

 fund. It is in mind to raise 820,000, but 

 the interest of any sum that may be con- 

 tributed can at once lie made available. 



" Statements wall be furnished from time 

 to time to contributors, keeping them in- 

 formed of the progi-ess of the contributions. 

 Checks may be made payable to the order 

 of Thomas Meehan, Director, or Steward- 

 son Brown, Treasurer, and mailed to either 

 at the Academy of Natural Sciences, Nine- 

 teenth and Race streets, Philadelphia."' 



THE MOTION OF CLOUDS. 



At a meeting of the Royal Meteorological 

 Society, of London, on March 20th, Mr. "W. 

 N. Sliaw, F. R. S., delivered a lecture on 

 ' The Motion of Clouds considered witli ref- 

 erence to their mode of formation,' which 

 was illusti"ated by experiments. The ques- 

 tion proposed for consideration was how far 

 the apparent motion of a cloud was a satis- 

 factory indication of the motion of the air 

 in which the cloud is formed. The moun- 



tain cloud cap was cited as an instance of a 

 stationary cloud formed in air moving 

 sometimes with great rapidity; gi-ound fog, 

 thunder clouds and cumulus clouds weie 

 also referred to in this connection. The 

 two causes of formation of cloud were next 

 considered, viz.: (1) the mixing of masses 

 of air at different temperatures, and (2) the 

 dynamical cooling of air by the reduction 

 of its pressure without supplying heat from 

 the outside. The two methods of formation 

 were illustrated bj- experiments. 



A sketch of the supposed motion of air 

 near the centre of a cyclone showed the 

 probabilitj- of the clouds formed by the 

 mixing of air being carried along with the 

 air after tliey formed, while when cloud is 

 being formed bj' expansion circumstances 

 connected with the formation of tb'ops of 

 water on the nuclei to be found in the air, 

 and the maintenance of the particles in a 

 state of suspension, make it probable that 

 the apparent motion of such a cloud is a bad 

 indication of the motion of the air. After 

 describing some special cases, Mr. Shaw re- 

 ferred to the meteorological efltcts of the 

 thermal disturbance which must be intro- 

 duced by the condensation of water vapori 

 and he attributed the atmospheric disturb- 

 ances accompanying tropical rains to this 

 cause. The difference in the character of 

 nuclei for the deposit of water drops was 

 also pointed out and illustrated bj' the ex- 

 hibition of colored halos formed under 

 special conditions when the drops were 

 sufficiently uniform in size. 



the discrimination of colors. 



Professor Arthur Konig (Zeit^chrift fur 

 Fsychologie, Feb., 1895) has ciilculated, from 

 experiments previously published, the num- 

 ber of hues or colors that can be distin- 

 guished in the spectrum. Differences in 

 hue cannot be perceived beyond / = G55 /i/t 

 and beyond / = 4.30 fift ; between these 

 limits the normal eye can distinguish about 



