Amongst the Echinodermata, which include the star-fishes, sea-hedgehogs, &c., there are many whose outer surface is covered with “spines,” or thin projections. Some of these are sharp and thorn-like, others blunt, longer or shorter, and, indeed, of endless variety. In many of these, when a section is made, rings are seen which have a common centre, with radiating supports, resembling sections of some of the woods. These are very beautiful objects, and methods of procuring them may now be considered. It is the best to cut as thin a section as can safely be got with the watch-spring saw first, when the smooth “sharpening stone” may be used to polish one side, which is easily accomplished with water only. When this is effected, it must be washed clean, and _thoroughly_ dried, and then may be united to the slide in the same manner as before recommended for the Orbitolite, &c. If it is ever necessary to displace it on account of inequalities, bubbles, or other remediable fault, this may be done by warming the slide; though too much heat must be avoided, otherwise fresh bubbles will certainly be produced. The covering with thin glass, balsam, &c., will present no difficulty to the student; but he must remember that the transparency is somewhat increased by this last operation. _Corals_ are often treated in this way, in order to reveal their structure. Except, however, the student has had much practice, he will often find this a most difficult task, as many of them are exceedingly brittle. He will find the method before described equally applicable here, and should take both horizontal and vertical sections. COAL.--This substance is one of the most interesting objects to the microscopist. It is, of course, of a vegetable origin; and though this is in many cases of such minute separate portions as to have lost all appearance of vegetation, yet it is very frequently met with in masses, bearing the form, even to the minute markings, of wood, in various directions. To see this and prepare it for microscopic research, a suitable piece of coal must be obtained; but in every case the cutting and preparation of these sections require great care and skill. Sometimes the coal is first made smooth on one side, fastened to the glass, reduced to the requisite degree of thinness, and finished in the method before described. This mode of treating it is sometimes, however, very tantalizing, as, at the last moment, when the section is about thin enough, it often breaks up, and so renders the trouble bestowed upon it fruitless. The dark colour and opacity of coal render an extraordinary thinness necessary, and so increase the liability to this accident.