December 1, 1899.] 



KNOWLEDGE. 



285 



Here the tail ia massive and muscular as in any lobster or 

 crawfish. But it is something more. It is the home of 

 the respiration. On its swimming-legs, filamentous 

 branchiffi are developed, adding one more singularity to the 

 many which distinguish the appearance and character of 

 this strange set of animals. 



In having a caudal system of respiration, the Squillidae 

 are companions of a widely difi'erent, and far more exten- 

 sive and varied division, the Isopoda. These are dispersed 

 over all oceans. They track the footsteps of mankind in 

 every land, or scale the mountains and explore the 

 wilderness in advance. To them the common woodlouse 

 belongs, and many an uncommon woodlouse. By their 

 sessile eyes, and their short jackets, and often by the 

 general aspect of their legs, they seem to be akin to the 

 Amphipoda, and with them they have another link of 

 connexion in the simplicity of their breathing organs. 

 Nevertheless, those organs in the Isopoda, instead of 

 belonging to the legs of the trunk, belong to the pleopods, 

 or legs of the tail. 



By name and nature, size and shape, a woodlouse makes 

 little appeal to our romantic feelings. It may claim 

 respect for antiquity of lineage. In some species it shows 

 a willingness to march with the times, by adapting its 

 branchiffl to a sort of tracheate respiration, such as suits a 

 subaerial, as distinct from an aquatic existence. But by 

 extraordinary variety of form and structure, its oceanic 

 cousins, the Isopoda in general, afi'ord a study of remarkable 

 interest. They are seldom, it is true, very large, con- 

 descending sometimes to the twentieth of an inch, though 

 ranging at others up to three or four inches of length by 

 an inch of breadth. TUl about twenty years ago, these 

 were the monsters of the isopod order. But then, to the 

 astonishment of carcinologists, there was hauled up from 

 a depth of about one thousand fathoms, Bathynomits 

 gic/antem, four inches broad and more than nine inches 

 long. It is a true isopod, with nearly four thousand facets 

 in each of its great sessile eyes, and with the normal 

 appendages according to rule. Only in one point it is 

 exceptional. To assist the breathing in this great deep- 

 ranging carcase, arborescent branchial appendages have 

 grown out from the laminse of its pleopods, parallel after 

 a fashion to what has been described in the Squillida?. 

 So, in various ways, does Nature provide for the welfare 

 of all her children. So thinking, Linnfeua prefixed to his 

 Systema, the saying, " .Jehova, quam ampla sunt Tua 

 opera ! quam sapienter ea fecisti ! quam plena est terra 

 possessione Tua ! " 



By John H. Cooke, f.l.s., f.g.s. 



To the current issue of the Transactions of the Botanical 

 Society of Edinburijh, Mr. R. A. Robertson contributes a practical 

 paper on the " Photomicrography of Opaque Stem Sections — 

 Recent and Fossil.' Mr. Robertson's method has much to re- 

 commend it to all who are engaged in research work or teaching, 

 and who need a method of reproducing accurately the diagnostic 

 characters of timber as seen in transverse and longitudinal 

 sections. Micro-sections as usually made do not present enough 

 surface to exhibit, typically, all of the characteristic appearances. 

 Large areas are therefore necessary, and it is to a demonstra- 

 tion of the best method of obtaining these that this paper is 

 devoted. No sectioning is necessary, as only the specially 

 dressed opaque surface of the block of wood is micrographed. 

 This is a distinct advantage where museum specimens are con- 

 cerned, as the micrograph may be taken without in any way 

 damaging the preparation. 



A photo-micrographic camera, which is capable of working 

 either vertically or horizontally, is best adapted for the purpose. 

 The microscope accessories are removed and a half-plate lens is 



fixed to the front of the camera. The wood specimen is 

 propped up in position in front of the camera so that the plane 

 surface is at right angle to the optio axis of the apparatus. 

 Focussing is arranged by means of a Welsbach incandescent gas 

 light, which stands on one side of the apparatus, and finally by 

 means of a focussing glass and magnesium wire. The radiant 

 for exposure is a magnesium ribbon apparatus, arranged like the 

 Welsbach on the opposite side. Mr. Robertson's experiments 

 show that eight inches from the surface of the wood is the best 

 position for the light when exposing. The time of exposure 

 varies with the plate, intensity of light, and colour of the 

 wood. With slow Ilfords forty seconds has been found to give 

 very satisfactory results. The accompanying micro-photograph 

 of a transverse section of a stem of Acacia catechu was taken 

 by this method by Mr. Robertson, 



Micro-Photograpli of Acacia catechu (transverse section). 



For obtaining photo-micrographs of smaller areas of surface 

 under a somewhat higher magnifying power, a microscope is 

 used with a low-power objective in place of the photographic 

 lens. A leus fitted with an iris diaphragm, on the same principle 

 as a photographic lens, is the best adapted for the purpose. The 

 ocular is removed, and a tube of mat-black paper is substituted. 

 The microscope is then bent over into a horizontal position, and 

 connected with the camera. Illumination, focussing, and ex- 

 posure are the same as before, but greater care, if possible, is 

 required. 



'The practical value of a knowledge of common crystals, and 

 of the modifications that they undergo when subjected to 

 various modes of treatment, has been frequently brought home 

 to the chemist and mineralogist when engaged on the determina- 

 tion of unknown substances. No microscopist can consider 

 himself fully equipped for his work who has not made himself 

 familiar with the commoner kinds of crystals prepared from 

 alcoholic and aqueous solutions, saturated and dilute ; by 

 spontaneous drying and crystallization ; and by rapid crys- 

 tallization with the aid of heat. It was whilst pursuing the 

 study of this branch of microscopical research that Dr. Sorby 

 made some of his most startling discoveries. One of these was 

 the determination of the nature of the liquid which fills the 



