April 20, 1888.] 



SCIENTIFIC NE^A^S. 



381 



or ordinary dry plate), which must be reversed. After 

 proper exposure, according to the density of the nega- 

 tive, the plate is thoroughly washed in water and dried ; 

 only a faint trace of the image or picture can be seen on 

 the plate at this stage. It is now placed in a printing 

 press (generally lithographic) damped with water, and a 

 leather roller charged with printing ink passed carefully 

 over the plate. 



Now is seen most clearly the curious and interesting 

 action of light upon the prepared gelatine — the gelatinous 

 surface receives the printing ink from the roller in exact 

 proportion to the amount of light that has passed through 

 the different parts of the negative, i.e., where there are 

 blacks in the picture the plate inks up solid ; where 

 white, it will take no ink at all ; and where half-tones 

 exist, it takes ink in the same gradation all over the 

 plate. It only remains now for the printer to lay a sheet 

 of printing-paper, smooth or rough, upon the plate and to 

 pass it through the press. The result will be a Collotype 

 print. One can also print excellent copies of any subject 

 upon calico, linen, satin, and other fabrics. A good plate 

 will give 400 to 500 copies, sometimes 1,000. Ink of any 

 colour may be used and any paper. The prints may be 

 varnished and mounted like ordinary photographs, or 

 they can be printed on paper with white margins, like an 

 engraving. 



With these few remarks I conclude my paper. These 

 arts, but briefly described, which have, as I have stated, 

 numerous allies, open up a field for further advancement 

 beyond the limits of ordinary speculation. 



They already have created an industry which capital 

 has not been slow to encourage, and which gives em- 

 ployment to thousands where only twenty years ago but 

 a few were earning by it a livelihood. 



Through these means our homes are enriched with 

 true copies of works of art, and at a low cost ; even the 

 poor man has within his reach illustrations with which 

 to cover the walls and educate his taste. And who shall 

 gainsay that through photography future generations may 

 receive the greater blessings that accrue to human life 

 by the mind being raised to higher and better aims 

 through the refining influence of art and knowledge ? 



From the Camera and its allied arts, cheap and faithful 

 pictures are placed within our reach, of art in its highest 

 forms, of the beauties and the wonders of the world we 

 live in, of our mountains, rivers, seas, and lakes, our 

 cities in countries of every clime. Nature in all its 

 grandeur, life in all its detail, is spread at our feet. 



ENEMIES OF TREES. 



WE see sometimes, says M. de Kerwan in Cosmos, 

 in autumn, in winter, and even in summer, 

 young shoots or ends of branches dropped by trees, 

 such as pines and firs, etc. 



In certain cases this fact is a trifling accident which 

 does not involve any serious consequences. In others 

 it is the symptom of a serious disaster which may ex- 

 tend to large masses, even to entire forests. Trees, 

 both the soft-leaved and the conifers — needlewood, as the 

 latter are aptly called in Germany — in addition to the 

 enemies common to all plants, have their special foes, 

 exceedingly formidable, the presence of which should be 

 early detected in order that they may be promptly com- 

 batted. 



It may happen that rodents, especially squirrels, are 

 to blame, and it is easy to ascertain this point by exami- 



ning the young shoots or sprays which have fallen off. 

 If the fracture of the twig presents a more or less dis- 

 tinct section without any aperture indicating the begin- 

 ning or the end of a minute tunnel, the shoot has been 

 cut off by the teeth of some rodent, or perhaps it may 

 have been detached by the beak of a rook or crow. In 

 this case, which is the least unfavourable, the evil is 

 merely transitory. Trees thus attacked suffer merely 

 the loss of a small portion of their leaves, and in the 

 following spring the injury is quickly repaired. 



But if the base of the young shoots where broken off 

 presents the aperture of an excavated gallery, whether in 

 the woody tissue itself or between the wood and the 

 bark, the evil is more serious, and may be hard to deal 

 with. It may be evidence that the trees thus attacked 

 contain in their branches, or even in their stems, 

 colonies of minute Coleoptera, of the genera Bostrichus, 

 Hylesinus and Scolyius. Certain species of these small 

 beetles penetrate into the wood in spring, and excavate 

 there galleries in which the females deposit a great 

 number of eggs. From these there quickly emerge 

 small larvae, which continue the ravages of their mothers. 

 When they have in turn arrived at maturity, each digs 

 out a small round hole of the diameter of its body, carry- 

 ing it often to the very tip of the shoot. This destruc- 

 tive work is continued until the Virinter. The insect^ 

 say Hylesinus piniperda — then leaves the young shoots, 

 which show outwardly nothing of the ruin within, and 

 hides itself at the foot of the trees, in the moss, in the 

 grass, or sometimes in the trunk of the tree. As the 

 winter comes on the perforated shoots gradually wither, 

 and fall to the earth under the weight of the frost or 

 snow, or from the strain of the wind. 



We have thus described the operations of the Hyle- 

 sinus of the pine-tree. But the family is rich in species. 

 There is a Hylesinus niger, Polygraphus pubescens, Tomi- 

 cus micrographus, Monograplius and Typographus, con- 

 sidered by some as the very worst, Scolytus destructor, 

 and several others. 



In different forms, and at different seasons of the year, 

 these pests all run their galleries, either in the wood or 

 between the wood and the bark. On stripping off the 

 bark from a decaying tree these galleries may often be 

 seen presenting a resemblance to written or printed 

 characters. 



Some begin their attacks in the branches, and these 

 are the least dangerous, because as soon as their 

 presence is detected, the branches may be cut off 

 below the point of attack and carefully burnt, thus de- 

 stroying the entire colony. Others penetrate at first into 

 the stem, and only appear in the branches after they 

 have ravaged the entire tree. The damage is then 

 beyond recovery, and all that can be done is to cut down 

 the infected trees and burn them, wood, bark, and twigs. 



The soil should be carefully raked around the infected 

 trees, and all the rubbish collected should be burnt. 



Black Snow. — M. Pezons, the cure of Saint Bandile, 

 writes to Cosmos that the country was already covered 

 with a deep layer of ordinary white snow, when a cold west 

 wind brought black snow in the form of small crystals. 

 This new layer, though very slight, gave the snow a very 

 distinct deep grey colour. The icicles adhering to the 

 trees were also blackish. This phenomenon was ob- 

 served over the space of about five square kilometres 

 (three square miles). 



