56 On Dark-ground Illiunination, [Sess. 



interesting to find such a near variety to the true Ccrastiiim 

 arcticum Lange as the specimen upon the table, from a station 

 much farther south. 



Alsine hirta var. foliosa, Hartm., from Serpentine Hills, Unst, 

 is very close to Alsine rubella. Dr Lange, in his 'Flora 

 Groenlandica,' p. 24, places A. hirta and A. rubella as mere 

 varieties of A. verna, which grows so plentifully on Arthur's 

 Seat, and which some of you may know better as Arenaria 

 verna. 



Luzula maxima D.C., var. gracilis Eostrup, was gathered by 

 Mr W. H. Beeby on Saxa Vord Hill, Unst, on the 28th July 

 1886, and appears to be an entirely new British plant. It is 

 found abundant on the upper slopes of the hill, but flowers on 

 the small plateau which forms the summit. 



Psamma baltica E. & S. This plant has been previously record- 

 ed from only one locality in Britain — namely, the Holy Isle and 

 adjoining coast of Northumberland. The specimen before you 

 is from a second British station — Caistor, in East Norfolk. 



Festuca sciuroides var. intermedia Hiichel med. This is a 

 new variety, recorded July 1886, from Mitcham Common 

 Surrey. Mr Bennett remarks, "It appears to be a connecting 

 form between Festuca sciuroides and F. myurus L." 



■XI.— ON DARK-GROUND ILLUMINATION, AS SHOW- 

 ING PECULIARITIES OF CERTAIN DIATOMS 



By Mr WILLIAM PENMAN, Assoc. M.Inst.C.E. 



{Read March 23, 1887.) 



Dark - ground illumination must not be confounded with 

 opaque illumination. The latter takes its name from the man- 

 ner in which the object viewed is mounted — that is to say, the 

 object is mounted upon an opaque ground, and treated with super- 

 stage illumination. Dark-ground illumination, on the other 

 hand, deals with mounts through which light can be trans- 

 mitted from beneath in the usual way. Besides this, however, 

 there are a few distinctions that must be attended to in the 

 selection of suitable objects for this treatment. Briefly, they 



