96 THE MICROSCOPE. 



glass, with the edges ground, such as are used for mounting objects ; 

 and lastly, a good and pretty powerful hand-magnifier. Two Cod- 

 din gton lenses, mounted in one frame of about half an inch, and one- 

 tenth of an inch in focal power, are specially convenient. 



Perhaps it will be as well, in pointing out a few localities, to de- 

 scribe some that I have actually visited, with the means of access, and 

 the appearance of the various species, en 'masse, that I have met with. 



Swanscombe Salt-marsh will be found well worth a visit ; and it 

 can be reached by steam-boat or railway from London-bridge to North- 

 fleet. On quitting the railway-station, make towards the almshouses 

 on the top of the hill j and arriving at the road, turn to the left, descend 

 the hill, and cross a sort of bridge over a somewhat insignificant 

 stream. Continue along the main -road a little farther, to a point 

 where it begins to ascend again, and diverges to the left towards the 

 railway j here quit it, taking your course along an obscure road, nearly 

 in a direct line with the main one ; passing a windmill on the right 

 hand, and continuing until you arrive at another still more obscure 

 road, turning off to the right ; which road appears as if made of the 

 mud dredged from the bottom of the river, and partially hardened. 

 This is Swanscombe Salt-marsh ; and the road just described leads 

 towards Broad Ness Beacon. On either side is a sort of ditch ; one 

 containing salt or very brackish water, the other filled with a sort of 

 black mud, about the consistence of cream, the surface being in parts 

 of a slaty grey, with little patches here and there of a most brilliant 

 brown colour, glistening in the sunshine, and presenting a striking con- 

 trast to the sombre shade. By carefully insinuating the end of one 

 of the slips of glass under this brilliant brown substance, and raising 

 it gently, it can be examined with the Coddington ; and it will pro- 

 bably be found to consist of myriads of specimens of Pleurosigma (na- 

 vicula of Ehrenberg) angulatum, or balticum, or some other species of 

 this genus. The iron spoon is now useful, as by its aid the brown 

 stratum, with little or no mud, can be skimmed off and bottled for 

 future examination. On the surface of the water in the other ditch 

 may be noticed a floating mass of a dark olive colour, which to the 

 touch feels not unlike a lump of the curd of milk, and consists of Cyclo- 

 tella menighiniana, and a surirella or two embedded in a mass of Spi- 

 rulina hutchinsia ; and another mass of floating weed, which feels harsh 

 to the touch, proceeding from a quantity of a synedra, closely investing 

 a filamentous alga j and elsewhere Meloseira nummuloides [gallionella of 

 Ehrenberg). 



In a trench by the sea-wall, as it is termed, is a mass of brown 



