34 Optical Ghosts. 



it (and which, are known at this day to inhabit only the extreme 

 north) could have lived in the same latitude as the south of 

 Sweden. The evidence of these fresh-water lakes suggests 

 that similar changes in the relative position of sea and land may 

 have been the cause of our having fresh- water Crustacea nearly 

 allied to marine species in our rivers and inland streams/' 



Higher in the scale of life the inhabitants of salt marshes 

 are few and far between ; a few sticklebacks and an occasional 

 gasteropodous mollusc of some common species will almost 

 exhaust the list. We should not, however, pass entirely with- 

 out mention a very interesting nudibranchiate mollusc, which 

 has been found in a few places. This is Alderia modesta, a 

 pretty little species of a greenish colour, living chiefly among 

 the tufts of Vaucheria, upon which it feeds. Where it occurs at 

 all it is mostly in great abundance ; but the only British localities 

 hitherto recorded are Lougher Marsh, near Swansea, a marsh 

 near Cork, and Hylton Dene, near Sunderland. Associated 

 with it may sometimes be found a little black slug of the genus 

 Limapontia. 



Salt marshes such as these, whose inhabitants have been the 

 subjects of our paper, are perhaps the nearest analogues which 

 our islands can now exhibit of those extensive lagoons which, 

 under the fostering influences of an almost tropical climate, 

 supported the dense forests of the Carboniferous period. It has 

 been inferred from certain animal remains found in the coal 

 strata, that those lagoons must have been, in some cases at 

 least, brackish ; but considering the widely different aquatic 

 conditions under which it has been shown that the same species 

 may exist, too great caution can scarcely be exercised in the 

 application of any evidence derived from fossil remains. 



OPTICAL GHOSTS. 



The old mode of obtaining spectral illusions by means of con- 

 cave mirrors presented many difficulties, which were practically 

 insuperable when (lie images were required to be on a large 

 scale, and to be comparable in Bharpness and apparent density 

 wit.li actual and similar objects seen at the same time. Lately 

 these difficulties have been wonderfully overcome, as the 

 " Patent Grhoifcs" exhibited at the Polytechnic, and elsewhere, 

 abundantly testify. So greal has been the popularity of these 

 exhibitions that, now the mystery is out, and an explanation is 

 offered by Mr. Dircks to the public, the book* purporting to 

 ,il l lie secret would have been widely welcomed had it 



* T/tf! QKott, as produced in the, Spectre ])rama, Popularly Illustrating the 

 Marvi -11' .us Optical Illusions, called tho Dircksian Phantasmagoria, by Henry 

 Dircks, C.E. Shaw. 



