IOO 



HARDWICKE\S SCIENCE-GOSSIP. 



essentially " wild." That is, they are not modified 

 like cultivated plants, nor so dependent on man's 

 exertions ; but may there not be exceptions ? Suppose 

 a third class of weeds, which, accompanying cultiva- 

 tion, have themselves been in some sense cultivated ; 

 may not this explain some anomalies ? And, in regard 

 to this Portulaca oleracea, we find three varieties 

 advertised for sale in France.* 



It is astonishing how readily weeds become acclima- 

 tised to new conditions. It might reasonably be 

 argued that, of the plants introduced into a country 

 by commerce, the majority would naturally be weeds. 

 Quite so, but this does not perhaps fully explain the 

 great adaptability and spread of weeds. Weeds, in 

 fact, are like freshwater productions living in isolated 

 water-system. f Their very existence as successful 

 weeds depends on their power of acclimatisation ; 

 hence great variability and little fixity of variations. 

 Cultivated plants would be just the same but for the 

 effects of artificial selection. 



In a new country, before any cultivation has taken 

 place, it is not always easy to say which of the wild 

 plants will prove aggressive weeds. A cultivated 

 patch of land called Smith's Park, somewhere over 

 nine thousand feet above sea level in the mountains 

 of Custer Co., Colorado, is instructive in this respect. 

 Chenopodium album puts in an appearance there in 

 great force, as if that was its own native home ; but 

 of the native plants we see Troximon glaucum repre- 

 senting the dandelion, and Echinospermum flori- 

 bundum and E. Redowskii representing themselves, to 

 a deplorable extent. These Echinospermums have 

 prickly fruits which get on one's trousers and on to 

 horses' manes and forelocks, and are thus productive 

 of considerable bad language — and this not from the 

 horses, who suffer most. E.floribundutn has pretty 

 pale-blue forget-me-not-like flowers, and its fruit 

 takes particular care that you don't forget it, being 

 the worse of the two. 



Sometimes weeds get carried above their natural 

 altitude by accident. Near the Micawber Mine, 

 about 10,000 feet above sea level in Western Custer 

 Co., Colorado, I noticed this. Clebme integrifolia 

 had come up from some dropped seed, but it certainly 

 would not propagate itself so high in the mountains, 

 and Helia?it/ius peliolaris, even more out of place, 

 doubtless owed its advent to seeds in some hay from 

 Pueblo County, about sixty miles away and thousands 

 of feet lower down. 



The study of weeds from the economic standpoint 

 is of more interest than might at first appear, not 

 to speak of its practical value. May I here com- 

 mend the matter to the increased attention of 

 botanists ? 



T. D. A. COCKERELL. 

 West Cliff, Custer Co., Colorado. 



* Vilmorin-Andrieux & Cie., " Cat. General de Graines," &c, 

 1889. p. 64. 

 f SciENCE-Gossir, August 1888, p. 183. 



D 



A DARK-FIELD STOP. 



By William Lightox. 



ARK-FIELD illumination, when using lenses 

 of high power, and especially homogeneous 

 immersion lenses, has been long desired and is at 

 last accomplished. 



After arranging the mirror so as to obtain central 

 light and removing the eye-piece, on looking down 

 the tube the mirror appears a bright figure in the 

 centre of the back lens of the objective (see Fig. 53, B). 

 Let the large circles (Figs. 53 to 58) represent the 

 back lens of an oil immersion or dry objective of large 

 aperture. On swinging the mirror from right to left 

 its image in the lens will pass from left to right, as 



Fig- 53- 



Fig. 54- 



Fig- 55- 



Fig. 56. 



Fig. 58. 



indicated in figures 55 and 56. Light from the mirror 

 in this position is known as oblique light. If, when 

 using a dry objective, the mirror is swung so far to 

 the left that its image cannot be taken up by the 

 objective, dark-field illumination is obtained (see 

 Fig. 57, E). 



When homogeneous-immersion objectives of large 

 numerical aperture are used, dark-field illumination 

 by the mirror alone is impossible, because such lenses 

 receive light from all points beneath the stage. 



The following method produces a dark-field with 

 the mirror in any position from central to extremely 

 oblique : 



A metal frame is used as a carrier for the dark-field 



