Were the line that joins the two poles vertical, the system would be perfectly astatic in a uniform field, since each magnet in particular would then be perfectly astatic. A pair of horseshoe magnets may thus be regulated in such a way as to form a perfectly astatic system in a uniform field and to preserve an almost invariable zero, this being something that it is very difficult to obtain with the ordinary arrangement of needles, especially when a compensating magnet is used; for, in such a case, one of the needles becomes more or less magnetized, while the other becomes demagnetized, according to the position of the compensating magnet.—La Lumiere Electrique.
HISTOLOGICAL METHODS.
A cat, dog, rabbit, or Guinea pig will furnish parts from which sections can be cut for the study of histology. Whichever animal is selected should be young and well developed. Put it under influence of chloroform, and open into the cavity of the chest; make an incision into the right ventricle, and allow the animal to bleed to death; cut the trachea and inject the lungs with a solution of one and a half drachms of chromic acid in one quart of water, care being taken not to overdistend the lung. Tie the severed end to prevent the escape of the fluid, and carefully remove the lung. It is a difficult thing to do this without rupturing it, but with care and patience it can be done. Place the lungs in a solution of the same strength as used for injecting; after fifteen or twenty hours change it to a fresh solution, and allow it to remain for about a month, and then change it to rectified spirits, in which it may remain until required.
Cut the tongue into several transverse and longitudinal pieces, also the small intestines, and put them into a solution of fifteen and one-half grains chromic acid, thirty grammes bichromate of potash, and three pints of water; change the solution the next day, and let them remain two weeks and then place in spirits. Cut longitudinal and transverse portions of the stomach and large intestines, wash in a weak solution of salt and water, and put them in the same solution as used for the lungs, and treat similarly.
Cut the kidneys longitudinally and transversely, and put them in a solution of six and one-half drachms bichromate of potash, two and one-half drachms sodium sulphate, one quart of water; change the solution the next day, and at the end of four weeks transfer to alcohol. Wash the inner surface of the bladder with salt and water, and after cutting it longitudinally and transversely, put the sections in a solution of three drachms bichromate of potash in a quart of water. Cut the liver into small parts, and place in the same solution as used for the kidneys; change the solution after a day, and let them remain four or five weeks, then change to spirits. The spleen and portions of the thin abdominal muscles may be placed in a solution of three drachms chromic acid to one quart of water, and transferred to alcohol after three or four weeks. Carefully remove an eye and divide it behind the crystalline lens, put the posterior portion in a solution made by dissolving fifteen grs. chromic acid in five drachms water, and slowly adding five and one-half ounces alcohol; change to spirits in two weeks. The lens should be put in the same solution, but should remain a few days longer. Open the head, remove the brain, and place transverse and longitudinal sections of it in spirits for eighteen hours, then transfer to a solution of one drachm chromic acid in a quart of water, and let it remain until hard enough to cut. Place the uterus in a solution of one and one-half drachms chromic acid in one quart of water, change to a new solution the next day, and at the end of a month transfer to alcohol.
The bones from one of the legs should be carefully cleaned of its muscles, cut into several pieces, and placed in a solution of fifteen and one-half grains chromic acid, one-half drachm nitric acid, and six ounces water. Change the fluid frequently until the bones are sufficiently softened, and then change to alcohol.
Section cutting machines for cutting sections can be procured of the dealers, but a very simple and effective one can be easily made if one does not wish to go to the expense of buying an instrument.
A strip of wood twelve or fourteen inches long and about two inches wide has attached to its center a bridge-shaped piece of wood, a, Fig. 1. This is covered with a brass plate, c, pierced with a hole one-half of an inch in diameter. This hole extends through the wood, and is fitted with a piston. Two long narrow inclined planes of nearly equal inclination, b, b, grooved to slide on each other, are placed under the bridge; the lower is to be fastened to the board; the end of the piston rests on the upper one. The object from which we desire to cut a section is placed in the hole, in the piston. If the upper plane be pushed in, the piston will be forced upward, and with it the object. As the inclination of the plane is very gradual, the vertical motion will be very slight as compared with the horizontal.