Page:The New International Encyclopædia 1st ed. v. 18.djvu/189

* SIGHT. 151 SIGISMUND. turned from tropical regions. It is frequent amuiig the natives of some parts of India, who attribute it, as sailors do, to sleeping exposed to the moonlight. The most probable cause ol the afl'ection is, however, exliaustion of the power of the retina from the over-excitement of e.xeessive liglit, so that this organ is rendered incapable of appreciating the weaker stimulating action of twilight or moonlight. tinoir-hliiidncss nuist be regarded as an allied affection to the preceding. Diiif-h!i)idn<-ss, or nyctalopia, refers to the con- dition in which the sight is better in a feeble light, as at dusk, than in bright light. This oc- curs in amblyopia (q.v. ) from the abuse of to- bacco, and in cases in which there is defective vision of the central portion of the visual field. For example: if there is an opacity of the central portion of the lens or cornea, the dilatation of the pupil whicii takes place in a feeble light al- lows the person to see through the unobstructed portion of the cornea or lens surrounding the opacity. Colored vision sometimes occurs either with or without retinal changes. Red vision, •rythropsia, occiu's after extraction of a cataract. Xanthopsia, or yellow vision, may follow the ingestion of san- tonin, gelsemium, digitalis, chromic and picric acids, and aniyl nitrate. Cannabis Indica some- times causes violet vision. Red or blue vision may result from the use of iodoform, and cocaine has caused colored vision. Phosphorus is said to cause sparks and flashes of light, and the same is said to be caused at times by. belladonna and santonin. Other defects of sight are described under the headings AM.vrRosis ; Amblyopia : Astigm.^- TisM : Hemiopia ; Heterophoria. See Vision. SIGHTS. The means by which cannon or small arms are pointed or aimed. There are almost as many varieties as there are varieties of weapons. With modern high-power guns, telescopic sights are necessar>' on ac- count of the difficulty of seeing the target at the ex- treme long range of which these guns are capable. The Scott telescopic sight, the invention of an English navy officer, with its va- rious modifications, is prob- ably the most generally used. In small arms two points are installed, one near each end of the bar- rel, so that when the rifle or pistol is brought to the position of firing the sights come readily into coinci- dence for the eye and en- able the aim to be directed at the object. The sights must represent the direct line in which the bullet is projected. It is evi- dent, therefore, that some form of adjustment is necessary if the sights are to be lised at more than one distance. In military rifles sufficient adjustment must be given to enable the aim to be accurately taken at any range up to 2.i00 yards. Military sights are all variations of one general type and usually consist of a leaf either Sighting Notch BIGHT OF U. 8. SPRING- FIELD KiFLE (1902), eeen from above. QLOBE FORESIGHT. lying flat or hinged upon a bed or block fixed to the barrel. The leaf must be raised to secure ad- ditional elevation, the distance being regulated by a sliding bar, in the centre of which a notch has been cut, through which the sight is taken over the tip of the foresight. The wind gauge is a ilevice which enables the marksman to direct his sight on the object aimed at, although the rifle is actually pointing to the right or left of the mark. As yet this device has only been used for long distance rifle - range matches and fine shoot- ing. Lateral adjustment is rarely necessary at the distances at which sporting rifles are used, and it is not univer- sally advocated as a feature of the military small arm, owing to the difficulty there would be in securing a transverse slide which shall not be so small and so stiff as to be worthless in the excitement of action, or when the soldier's fingers are cold. Sportsmen sometimes employ a foresight of the covered bead var- iety, or the ivory or copper sight used by African hunters. The Boers, mIio rank high as practical marks- men, employ the ivory foresight, but Euro- pean and American sportsmen who have engaged in African or big game shooting generally use the cop- per sight. Various forms of rear sight are in use, perhaps the best of which is the platina bar on a more or less open V. Another favorite type is the platina pyramid, which is set below "a very open V. Telescopic sights are affixed to sporting rifles, and liave been found very successful in deer- stalking or any form of hill and mountain hunt- ing where game is difficult to locate. See Gun- nery; Guns, Naval; Small Arms; Target Practice; Range-Findeb, etc. SIG'ILLA'RIA (Neo-Lat. nom. pi., from Lat. sir/illiiin, seal). An extinct genus of lycopods that flourished during the Carboniferous" period, forming a conspicuous element of the swamp flora of that time. They were trees that often grew to great heights and had few branches. Both branches and trunk were crowded with sword-shaped leaves which were arranged in spiral series. See Lepidodendron ; Stigmaria. SIGISMUND, sijls-mund. Ger. pron. ze'g^s- miint (c.l3f)8-1437). Holy Roman Emperor from 1411 to 1437. He was the second son of the Emperor Charles IV., whom he succeeded in 1378 in the Margraviate of Brandenburg. In 1379 he PEEP SIGHT.