INJECTION PROBE 



A distinction should be drawn, as well, be- 

 tween illumination and luminance. The latter 

 is a measure of the light coming from a surface ; 

 thus, for a surface which is not self-luminous, 

 luminance is entirely dependent upon the illumi- 

 nance upon that surface and its reflection 

 properties. (5) 



ilmenite — A mineral, FeTiOs-, the principal ore of 

 titanium. Sometimes mined from beach and 

 shallow water sand deposits. 



immobile ice — A Russian term for fast ice. 



impedance — The total opposition (in ohms) 

 offered to the flow of an alternating current. It 

 may consist of any combination of resistance, 

 inductive reactance, or capacitive reactance. 



incised — Pertaining to a steep-sided trench or 

 notch cut into a plane surface or slope by cur- 

 rent erosion, as a sea valley or submarine can- 

 yon cut into a continental shelf or slope. 



inclination — (also called dip). In terrestrial 

 magnetism, the angle which the total magnetic 

 field vector makes with its horizontal component. 

 (15) 



incoming solar radiation — See insolation. 



index contours — Certain contour lines (usually 

 every fifth) accentuated by use of a line heavier 

 than the intervening ones. ( 2 ) 



index of refraction — (or refractive index; also 

 called absolute index of refraction, absolute re- 

 fractive index, refractivity) . 1. A measure of 

 the amount of refraction (a property of a di- 

 electric substance) . It is the ratio of the wave- 

 length or phase velocity of an electromagnetic 

 wave in a vacuum to that in the substance. It 

 can be a function of wavelength, temperature, 

 and pressure. (5) 



2. (or refractive index; also called relative 

 index of refraction) . A measure of the amount 

 of refraction experienced by a ray as it passes 

 through a refractive interface, that is, a surface 

 separating two media of different densities. It 

 is the ratio of the absolute indices of refraction 

 of the two media {see definition 1 above). (5) 



3. As related to ocean waves, it is the measure- 

 ment of the amount of refraction (or turning) of 

 an ocean wave front as the wave passes from 

 deeper into shallower water. It is a function 

 of wavelength, water depth, and the angle that 

 the approaching wave makes with the depth con- 

 tour. See wave refraction. 



Indian Ocean — That ocean area bounded on the 

 north by the southern limits of the Arabian Sea, 

 Laccadive Sea, Bay of Bengal, the limits of the 

 East Indian Archipelago and the Great Aus- 

 tralian Bight ; on the east from South East Cape 

 (the southern point of Tasmania) down the 

 meridian to the Antarctic Continent; and on 

 the west from Cape Agulhas southward to the 

 Antarctic Continent. 



The limits of the Indian Ocean exclude the 

 seas lying within it. 



Indian spring low water — The approximate mean 

 water level determined from all lower low 

 waters at spring tides. It is also the computed 

 plane located below mean sea level by an amount 

 equal to the sum of the amplitudes of the har- 

 monic constants M2, 82, K^, and 0-^. 



Indian tide plane— The datum of Indian spring 

 low water. (73) 



indicator species^A species of marine plankton 

 that is characteristic of a certain water mass to 

 which it is restricted, so that, with proper pre- 

 cautions, its presence can be taken as an indica- 

 tion of the presence of water of that origin. 

 Species of medusae, chaetognaths, euphausiids, 

 pteropods and tunicates, among others have been 

 shown to be indicator species. 



induced magnetism — The field produced in a 

 magnetic material when it is placed in an ambi- 

 ent magnetic field. In the earth's field, the in- 

 duced magnetism is in the direction of the field 

 and the strength is proportional to it. 



induced radioactivity — That radioactivity pro- 

 duced in a substance after bombardment with 

 neutrons or other particles. The resulting 

 activity is natural radioactivity if formed by 

 nuclear reactions occurring in nature and arti- 

 ficial radioactivity if the reactions are caused 

 by man. See nuclear reaction. (70) 



induration — The hardening of sediments through 

 cementation, pressure, heat, or other processes. 

 (2) 



inertia currents — Currents resulting after the 

 cessation of wind in a generating area or after 

 the water movement has left the generating area. 

 See swell. 



infralittoral — Below the region of littoral 

 deposits. (2) 



infraneritic — According to some authorities, 

 this term refers to the marine environment in 

 a zone between 120 and 600 feet (36 and 183 

 meters) below sea level. 



infrared — See infrared radiation. 



infrared radiation — (abbreviated IR; also called 

 long-ioave radiation or infrared). 'Electro- 

 magnetic radiation lying in the wavelength 

 interval from about 0.8 micron to an indefinite 

 upper boundary sometimes arbitrarily set at 

 1,000 microns (0.01 centimeter). At the lower 

 limit of this interval, the infrared radiation 

 spectrum is bounded by visible radiation, while 

 on its upper limit it is bounded by microwave 

 radiation of the type important in radar 

 technology. (5) 



initial angle — The angle that a sound ray leaving 

 a sound source makes with a horizontal plane. 



initial range — The horizontal distance between 

 the source and the closest point of the reswept 

 (convergence) zone. 



injection probe — A thermistor that is installed 

 in the ship's sea water injection intake pipe 

 (engine room). 



85 



