VIS 



VI V 



composition of gum Arabic, which ren- 

 ders the liquid ropy and thick, and hence 

 the application of the name viscous to 

 this fermentation. See Fermentation. 



VISION. In Optics, the sense of 

 sight ; this sense is produced by certain 

 nerves which convey to the brain intel- 

 ligence of figures impressed by rays of 

 light upon the retina of the eye. Short- 

 sightedness is occasioned by the con- 

 vergence of the rays to a point before 

 they fall upon the retina, and a concave 

 lens is employed to delay their conver- 

 gence. Long-sightedness is occasioned by 

 the non-conveyance of the rays to a point 

 until they have passed the retina, and a 

 convex lens is employed to promote their 

 convergence. 



1. Vision recrossed. A term applied 

 by Captain J. Grover to a distinct tribe 

 of ocular phenomena, in which objects 

 placed between and very near the eyes, 

 such as the two sides of the nose, appear 

 on opposite sides of the sphere of vision : 

 the object on the right side of the nose 

 being seen to the left by the right eye, 

 and that which is on the left of the nose 

 being seen to the right by the left eye. 

 It is supposed that, besides the two ex- 

 ternal or cranial eyes, *' man is endowed 

 with an internal cerebral organ, which 

 performs the office of a third eye, by 

 being the common recipient of impres- 

 sions propagated either from one or both 

 of the external eyes ; and the mind, in 

 her chamber of percipience. steers with 

 regard to external objects by the same 

 principle on which the mariner steers by 

 his compass. 



2. "Thus the two cranial eyes are 

 analogous, in principle and situation, to 

 two magnetic compasses placed upon a 

 ship's deck ; while the third, or cerebral 

 eye, corresponds to another compass 

 in the cabin below ; and the mind, situ- 

 ated like the captain-mariner in his 

 cabin, knows, from consulting the cere- 

 bral eye, on what point of direction the 

 body is steering ; although the mind no 

 more perceives either any external object, 

 nor yet any image in the cranial eye, 

 than the mariner perceives (even in the 

 vulgar sense of the word perceiving) the 

 far-off land, or haven, towards which he 

 is surely making his way." 



VISUAL {visus, sight). That which 

 accompanies vision : the visual angle is 

 the angle under which a body is seen ; 

 and the apparent magnitude of the same 

 object, when viewed at diifereut dis- 

 tances, depends on the size of the visual 

 869 



angle — that is, the angle formed at the 

 eye by the rays proceeding from the ex- 

 tremities of the object. See Theodolite. 



VITA'CE^ {vitis, a vine). The Vine 

 tribe of Dicotyledonous plants. Climbing 

 shrubs with tumid joints, and leaves sti- 

 pulate ; flowers polypetalous, on ramose 

 peduncles ; stamens hypogynous ; ova- 

 rium 2-celled ; fruit baccate ; seeds albu- 

 minous. 



VITAL AIR. A term applied to 

 oxygen gas, from its being indispensable 

 to the maintenance of life. It was for- 

 merly called dephlogisticated air, em- 

 pyreal air, &c. 



VITE'LLUS. Literally, the yoke of an 

 egg; a term applied by Goertner to a 

 fleshy sac occasionally interposed be- 

 tween the albumen and the ovule, in 

 plants, enveloping the latter. This sac 

 is usually referred to the vesicula amnios 

 of Malpighi. 



VITRIFICA'TION {vitrum, glass, fio, 

 to become). The conversion of a sub- 

 stance, as silica, into glass. 



VI'TRIOL {vitrum, glass). A term 

 originally applied to any crystalline body 

 possessing a certain degree of transpa- 

 rency, but now restricted to three sul- 

 phates — that of iron, called ferrous oxide, 

 copperas, or green vitriol ; that of copper, 

 or blue vitriol ; and that of zinc, or white 

 vitriol. 



1. When Green Vitriol is exp(^ed to 

 heat in a retort, it first gives off waler of 

 crystallization, ox phlegm of vitriol ; next 

 comes an acid, called spirit of vitriol; 

 then a stronger acid, called oil of vitriol ; 

 the latter part of this becomes solid, and 

 has been called glacial oil of vitriol. 



2. The stalactitic coh alt-vitriol, from 

 Herrengrund in Hungary, is sulphate of 

 magnesia, coloured red by oxide of co- 

 balt. 



VITTuE {vitta, a riband). A term ap- 

 plied, in Botany, to little clavate vessels 

 of oil found in the coat of the fruit of 

 umbelliferous plants ; when situated 

 in the valleculce, or depressions be- 

 tween the ridges of the fruit, they are 

 termed dorsal ; when on the face of the 

 fruit, commissural. They offer a special 

 case of the vasa propria, or receptacles 

 of secretion. Modifications of these 

 vessels are found in the leaves of the 

 orange and of all myrtaceous plants, 

 where they are called crypts, vesicular 

 glands, or receptacles of oil. See Vessels, 

 Milk, 4-c. 



VIVE'RRIDiE {viverra, the civet). 

 The civets ; a group of animals placed by 

 R5 



