Page:The American Cyclopædia (1879) Volume XI.djvu/539

 MICROSCOPE 521 cork, enclosed in a brass cylinder pierced with holes, to receive an insect pin, in case it be de- sirable to examine cabinet specimens. The forceps should have free movement in all direc- tions. The recent ones supplied by Beck have a ball and socket movement ; they are attached to the upper stage plate, and thus are moved by the rack and screw of the stage. The parabola was originally contrived by Mr. Wen- ham, and was composed of a silver reflector ; it is now made of glass, the outer surface hav- ing the form of a parabola. The rays of light, entering the glass through a plane surface below, suffer total reflection, and emerge with- out refraction, the upper surface of the trun- cated parabola being concave, so that each re- flected ray strikes upon the surface perpen- dicularly. The glass parabola has a small hole in the axis, carrying a sliding rod, with a small disk, which may be elevated or depressed ac- cording to the angle of aperture of the objec- tive, until the direct light is excluded ; the ob- ject is then exhibited entirely by oblique light on a dark field. The polycystina are beauti- fully shown by this illumination, and also parts of insects with the lower powers. The erec- tor consists of two plano-convex lenses, to be inserted into the lower end of the draw -tube ; by means of these the object is seen without being inverted. The magnifying power is much diminished, and the definition injured; and one who intends to dissect under the com- pound microscope, had better at once learn to manipulate without it. The ingenious erect- ing prism of Nachet may be used, which does not in the least injure the definition. In this case the upright stand of Nachet will be found very steady and convenient. The upright po- sition of the continental stand, with its short tube and low stage, is better adapted for real work, in dissection and in mounting objects, than the more elaborate English and American instruments ; while the latter are superior for the resolution of test objects and for the study of tissues already prepared and mounted under the low vertical instrument. Lieberkiihns are polished silver specula fitted to the lower powers, the polished surface being presented to the object; they are used in conjunction with what are termed dark wells, which con- sist of a slender rod carrying at the summit a blackened cap of brass, and are placed directly under the object so as to cut off the direct light ; where the object itself is mounted on an opaque background, they are unnecessary. The light reflected from the mirror below, falling upon the Lieberkiihn, is condensed upon the object. There are many opaque objects which cannot be well seen without this kind of illumi- nation, and it is justly considered a very im- portant addition to the apparatus. The frog plate is a flat mahogany or metal plate, having a slightly elevated glass platform, and the edge of the plate pierced with holes or furnished with split pins. By means of threads tied to the toes of the frog, the web may be spread out on the plate, the body of the frog being enclosed in a bag and strapped to the plate ; care must be taken not to draw the string too tightly around the mouth of the bag where the leg protrudes, as it stops the circulation. The bag may be dispensed with by holding the frog a moment or two in water of 120 F., when it becomes perfectly rigid and apparently insensible. In this condition it may be opened, and the circulation of the blood in the veins and arteries of the mesentery most beautifully exhibited ; care must be taken to keep the parts moist. For minute dissection we have found the eye instruments exceedingly useful. Fine scissors and forceps are made for this purpose, of most excellent quality, by Char- riere of Paris. In working with the monocu- lar microscope, one should acquire the habit of keeping both eyes open. If the eyes are allowed to rest easy, and then the focus care- fully adjusted to. suit while thus resting, no great fatigue will be experienced in using the microscope ; if, however, the focus is only ap- proximated, and then distinct vision obtained by forcing the eye, severe pain and headache will ensue. The writer has worked with a bright light for six or eight hours, with but slight intermission, without fatigue. Long ex- perience and great caution are requisite in in- terpreting the phenomena revealed by the mi- croscope, and one soon learns that things are not always what they seem. This, however, cannot be justly urged against its use. When directed by skilful hands, and guided by expe- rience, its revelations are of the most exalted and truthful character. No one can hope to succeed in the department of natural history who is not a skilful microscopist. A north light, coming from the left hand, the stage movements being at the right hand, is the best light for day. At night, the naphtha or coal oil lamp, as now made by Beck, gives the most intense and steady flame. The gas jet is difficult to manage for delicate work. A simple and very effective arrangement for illumination is that used by Dr. Edward Curtis of New York : "A small petroleum lamp is placed in a cigar box, which stands on one of its ends. On one side of the box is cut a small aperture, in which is placed a piece of blue glass, to soften the light as it passes to the microscope mirror. Another larger opening is made in the front of the box, and is occupied by three dif- ferent glasses. The one nearest the lamp is a square piece of ground glass ; the next one is also square and flat, but colored blue. Finally, a plano-convex lens of long focus is placed at such an inclination as to condense the rays of light, thus softened, on to the work-table for use in dissecting or in arranging prepara- tions." Many of the " test objects " have been so long known that it seems almost unneces- sary to mention them. The markings on the silicious shells of diatoms have been highly recommended. As general tests, they are no doubt very serviceable, but they are not en-