While engaged in the former of these employments--the searching of outlying rocks with the boat--and also when examining the outer rock pools which are disturbed by the waves that wash over their banks, the simple instrument known as the water-telescope will prove invaluable. Everyone must have noticed how difficult it is to observe objects in water, the surface of which is disturbed by the wind or some other cause; but the simple appliance named, consisting only of a long tube of metal, a few inches in diameter, and painted a dead black inside, will enable the observer to see all submerged objects with the greatest of ease when the water is itself clear. The principle of the water-telescope is as simple as its construction; for the tube, protecting the surface of the water within it from the disturbances outside, prevents the light from being refracted successively in different directions, while the dead-black surface of the interior prevents those internal reflections that would otherwise cause the vision to be indistinct. A few hours spent with the rhodosperms at the sea-side will be sufficient to show not only the great variety of their form and colouring, but also that the same species may vary according to the position in which it grows. Most of the smaller forms are delicate and filamentous, but others have expanded fronds which are very leaf-like. The brightest colours are usually to be found at or beyond low-water mark, where the weeds are covered with a considerable height of water for hours together, and also in shady situations at higher levels, while some of the species that grow in the upper rock pools are often of such a deep colour, with so much admixture of brown, that they may be easily mistaken for the olive melanospores to be presently described. Most of the rhodosperms are attached directly to the rocks, and the larger species have often a root-like disc by which they are very firmly held; but some of the smaller species grow attached to larger weeds, into the substance of which they frequently penetrate; and it is possible that these derive some amount of nourishment from the sap of their supporters. Some are of a recumbent nature, being attached to the rock throughout their whole length, while others are so incrusted with carbonate of lime which has been extracted from the water that they resemble corals rather than forms of vegetable life. Nearly all of them contain a bright-red colouring matter in addition to the chlorophyll by which they are enabled to feed on carbonic acid gas. None of the rhodosperms are of really microscopic dimensions, and they all grow by the repeated division of the cells of the apex, while the branches are derived by the similar division of new cells at the sides.