A piloted-simulation evaluation of two electronic display formats for approach and landing

The results of a piloted-simulation evaluation of the benefits of adding runway symbology and track information to a baseline electronic-attitude-director-indicator (EADI) format for the approach-to-landing task were presented. The evaluation was conducted for the baseline format and for the baseline format with the added symbology during 3 deg straight-in approaches with calm, cross-wind, and turbulence conditions. Flight-path performance data and pilot subjective comments were examined with regard to the pilot's tracking performance and mental workload for both display formats. The results show that the addition of a perspective runway image and relative track information to a basic situation-information EADI format improve the tracking performance both laterally and vertically during an approach-to-landing task and that the mental workload required to assess the approach situation was thus reduced as a result of integration of information

Spectro-microscopy of single and multi-layer graphene supported by a weakly interacting substrate

We report measurements of the electronic structure and surface morphology of exfoliated graphene on an insulating substrate using angle-resolved photoemission and low energy electron diffraction. Our results show that although exfoliated graphene is microscopically corrugated, the valence band retains a massless fermionic dispersion, with a Fermi velocity of ~10^6 m/s. We observe a close relationship between the morphology and electronic structure, which suggests that controlling the interaction between graphene and the supporting substrate is essential for graphene device applications.Comment: 10 pages of text, 4 JPEG figure

A Florida Soldier in the Army of Northern Virginia: The Hosford Letters

A long neglected area of American Civil War history has been the thousands of unpublished letters and diaries still found gathering dust in hundreds of attics across the nation. The recent Civil War centennial brought out some material of this type, and the general public has grown more aware of the advantages to be gained from the publication of soldiers’ memoirs. Despite this fact, it is rare indeed to find as moving and articulate a series of letters as the ten written by a young Florida Confederate officer, Lieutenant John W. Hosford, to his sweetheart, Miss Laura Rich, between November 1862 and April 1864. Hosford was a member of Company H, Fifth Florida Infantry, which was a part of General Robert E. Lee’s Army of Northern Virginia, and he participated in some of the hardest fighting of the entire war

The population of close double white dwarfs in the Galaxy

We present a new model for the Galactic population of close double white dwarfs. The model accounts for the suggestion of the avoidance of a substantial spiral-in during mass transfer between a giant and a main-sequence star of comparable mass and for detailed cooling models. It agrees well with the observations of the local sample of white dwarfs if the initial binary fraction is close to 50% and an ad hoc assumption is made that white dwarfs with mass less than about 0.3 solar mass cool faster than the models suggest. About 1000 white dwarfs brighter than V=15 have to be surveyed for detection of a pair which has total mass greater than the Chandrasekhar mass and will merge within 10 Gyr.Comment: 15 pages, 7 figures, to appear in Proc. ``The influence of binaries on stellar population studies'', Brussels, August 2000 (Kluwer, D. Vanbeveren ed.

Distribution and Taxonomy of Mammals of Nebraska

Contents Introduction Environment Climate Soils Geology and Physiography Vegetation Effects of Man on The Envmonment Factors Influencing Distribution And Speciation Wisconsin Glaciation And Post-Wisconsin Climates Emigration of Recent Mammals To Nebraska Some Effects of The Environment The Missouri River as a Barrier to Dispersal Geographic Varlatlon and Speciation Mammalian Distributional Areas Summary Treatment and Acknowledgments Checklist (with page references) of Mammals of Nebraska Accounts of Species Introduced Mammals Species of Unverified occurrence Type Localities Literature Cited ● Order MARSUPIALIA-Marsupials ● ● Family DIDELPHIDAE-Opossums ● Didelphis marsupialis virginiana Kerr (Opossum) ● Order INSECTIVORA-Insectivores ● Family SORICIDAE-Shrews ● Sorex cinereus haydeni Baird (Masked Shrew) ● Blarina brevicauda brevicauda (Say) (Short-tailed Shrew) ● Blarina brevicauda carolinensis (Bachman) (Short-tailed Shrew) ● Cryptotis paroa parva (Say) (Least Shrew) ● Family TALPIDAE-Moles ● Scalopus aquaticus caryi Jackson (Eastern Mole) ● Scalopus aquaticus machrinoides Jackson (Eastern Mole) ● Order CHIROPTERA-Bats ● Family VESPERTILIONIDAE-Vespertilionid Bats ● Myotis evotis evotis (H Allen) (Long-eared Myotis) ● Myotis keenii septentnonalis (Trouessart) (Keen\u27s Myotis) ● Myotis lucifugus canssima Thomas (Little Brown Myotis) ● Myotis lucifugus lucifugus (Le Conte) (Little Brown Myotis) ● Myotis subulatus stlbulatus (Say) (Small-footed Myotis) ● Myotis volans interior Miller (Long-legged Myotis) ● Lasionycteris noctivagans (Le Conte) (Silver-haired Bat) , ● Pipistrellus subflavus subf/avus (F Cuvier) (Eastern Pipistrelle) ● Eptesicus fuscus fuscus (Palisot de Beauvois) (Big Brown Bat) ● Eptesicus fuscus pallidus Young (Big Brown Bat) ● LasiUTUS borealis borealis (M tiller) (Red Bat) ● LasionycUTUS cinereus cinereus (Palisot de Beauvois) (Hoary Bat) ● Nycticeius humeralis humeralis (Rafinesque) (Evening Bat) ● Family MOLossIDAE-Free-tailed Bats ● Tadarida brasiliensis mexicana (Saussure) (Brazilian Free-tailed Bat) ● Order LAGOMORPHA-Hares, Rabbits and Pikas ● Family LEPORIDAE-Hares and Rabbits ● °Oryctolagus cuniculus (Linnaeus) (European Rabbit) ● SylviZagus audubonii baileyi (Merriam) (Desert Cottontail) ● Sylvilagus /loridanus mearnsii (J A Allen) (Eastern Cottontail) ● Sylvilagus floridanus similis Nelson (Eastern Cottontail) ● Lepus califomicus meZanotis Mearns (Black-tailed Jackrabbit) ● Lepus townsendii campanius Hollister (White-tailed Jackrabbit) ● Order RODENTIA-Rodents ● Family SCIURIDAE-Squirrels and Allies ● Tamias striatus griseus Mearns (Eastern Chipmunk) ● Eutamias minimus pallidus A Allen) (Least Chipmunk) ● Marmota monax bunkeri Black (Woodchuck) ● Spermophilus franklinii (Sabine) (Franklin\u27s Ground Squirrel) ● Spermophilus richardsonii elegans Kennicott ( Richardson\u27s Ground Squirrel) ● Spermophilus spilosoma obsoletus Kennicott (Spotted Ground Squirrel) ● Spermophilus tridecemlineatus pallidus J A Allen (Thirteen-lined Ground Squirrel) ● Spermophilus tridecemlineatus tridecemlineatus (Mitchill) (Thirteenlined ● Ground Squirrel) ● Cynomys ludovicianus ludovicianus (Ord) (Black-tailed Prairie Dog) ● Sciurus carolinensis pennsylvanicus Ord (Gray Squirrel) ● Sciurus niger TU/iventer :E: Geoffroy St-Hilaire (Fox Squirrel) ● Glaucomys volans volans (Linnaeus) (Southern Flying Squirrel) ● Family GEOMYIDAE-Pocket Gophers ● Thomomys talpoides cheyennensis Swenk (Northern Pocket Gopher) ● Thomomys talpoides pierreicolus Swenk (Northern Pocket Gopher) ● Geomys bursarius lutescens Merriam (Plains Pocket Gopher) ● Geomys bursarius majusculus Swenk (Plains Pocket Gopher) ● Family HETEROMYIDAE-Pocket Mice and Kangaroo Rats ● Perognathus fasciatus fasciatus Wied-Neuwied (Olive-backed Pocket Mouse) ● Perognathus fasciatus olivaceogriseus Swenk (Olive-backed Pocket Mouse) ● Perognathus flavescens flavescens Merriam (Plains Pocket Mouse) ● Perognathus flavescens pemiger Osgood (Plains Pocket Mouse) ● Perognathus flavus piperi Goldman (Silky Pocket Mouse) ● Perognathus hispidus paradoxus Merrlam (Hispid Pocket Mouse) ● Perognathus hispidus spilotus Merriam (Hispid Pocket Mouse) ● Dipodomys ordii luteolus (Goldman) (Ord\u27s Kangaroo Rat) ● Dipodomys ordii richardsoni A Allen) (Ord\u27s Kangaroo Rat) ● Family CASTORIDAE-Beavers ● Castor canadensis missouriensis V Bailey (Beaver) ● Family CRIcETIDAE-Native Rats and Mice ● Reithrodontomys megalotis dychei J A Allen (Western Harvest ● Mouse) ● Reithrodontomys montanus albescens Cary (Plains Harvest Mouse) ● Reithrodontomys montanus griseus V Bailey (Plains Harvest Mouse) ● Peromyscus leucopu,s aridulus Osgood (White-f~t~d Mouse) ● Peromyscus leucopus noveboracensis (Fischer) (White-footed Mouse) ● Peromyscus maniculatus bairdii (Hoy and Kennicott) (Deer Mouse) ● Peromyscus maniculatus luteus Osgood (Deer Mouse) ● PerOmYscus maniculatus nebrascensis (Coues) (Deer Mouse) ● Onychomys leucogaster arcticeps Rhoads (Northern Grasshopper Mouse) ● Onychomys leucogaster breviauritus Hollister (Northern Grasshopper Mouse) ● Sigmodon hispidus texianus (Audubon and Bachman) (Hispid Cotton Rat) ● Neotoma cinerea rupicola J A Allen (Bushy-tailed Woodrat) ● Neotoma floridana baileyi Merriam (Eastern Woodrat) ● Neotoma floridana campestris J A Allen (Eastern Woodrat) ● Synaptomys cooperi gossii (Coues) (Southern-Bog Lemming) ● Synaptomys cooperi relictus Jones (Southern Bog Lemming) ● Jrficrotus ochrogaster haydenii (Baird) (Prairie Vole) ● Microtus ochrogaster ochrogaster (Wagner) (Prairie Vole) ● Microtus pennsylvanicus finitus Anderson (Meadow Vole) ● Microtus pennsylvanicus pennsylvanicus (Ord) (Meadow Vole) ● Microtus pinetorum nemoralis V Bailey (Woodland Vole) ● Ondatra zibethicus cinnamominus (Hollister) (Muskrat) ● *Family MURIDAE-Old World Rats and Mice ● *Mus musculus Linnaeus (House Mouse) ● *Rattus floroegicus (Berkenhout) (Norway Rat) ● Family ZAPODIDAE-Jumping Mice ● Zapus hudsonius pallidus Cockrum and Baker (Meadow Jumping Mouse) ● *Family CAPROMYIDAE-Nutria and Allies ● *Myocastor coypus (Molina) (Nutria or Coypu) ● Family ERETlDZoNTIDAE-Porcupines ● Erethizon dorsatum bruneri Swenk (Porcupine) ● Order CARNIVORA-Carnivores ● Family CANIDAE-Coyote, Wolves and Foxes ● Canis latrans latrans Say (Coyote) ● Canis lupus nubilus Say (Gray Wolf) ● Vulpes velox velox (Say) (Swift Fox) ● Vulpes wIpes regalis Merriam (Red Fox) ● Urof!Yon cinereoargenteus ocythous Bangs (Gray Fox) ● Family URSIDAE-Bears ● Ursus americanus americanus Pallas (Black Bear) ● Ursus horribilis horribilis Ord (Grizzly Bear) ● Family PROCyoNIDAE-Raccoon and Allies ● Procyon lotor hirtus Nelson and Goldman (Raccoon) ● Family MusTELIDAE-Weasels, Skunks and Allies ● Mustela frenata longicauda Bonaparte (Long-tailed Weasel) ● Mustela frenata primulina Jackson (Long-tailed Weasel) ● Mustela frenata spadix (Bangs) (Long-tailed Weasel) ● Mustela nigripes (Audubon and Bachman) (Black-footed Ferret) ● Mustela nivalis cainpeStris Jackson (Least Weasel) ● M ustela vison letifera Hollister (Mink) ● Gulo luscus luscus (Linnaeus) (Wolverine) ● Taxidea taxus taxus (Schreber) (Badger) ● Spilogale putorius interrupta (Rafinesque) (Spotted Skunk) ● Mephitis mephitis hudsonica Richardson (Striped Skunk) ● Lutra canadensis interior Swenk (Otter) ● Family FELIDAE-Cats ● Felis concolor hippolestes Merriam (Mountain Lion) ● Lynx canadensis canadensis Kerr (Lynx) ● Lynx rufus rufus (Schreber) (Bobcat) ● Order ARTIODACTYLA-Even-toed Ungulates ● Family CERvIDAE-Wapiti, Deer and Allies ● Cerous canadensis canadensis Erxleben (Wapiti) ● Odocoileus hemionus hemionus (Rafinesque) (Mule Deer) ● Odocoileus virginianus macrourus (Rafinesque) (White-tailed Deer) ● *Dama dama (Linnaeus) (Fallow Deer) ● Family ANnLocAPRIDAE-Pronghorn ● Antilocapra americana americana (Ord) (Pronghorn) ● Family BovIDAE-Bovids ● Bison bison bison (Linnaeus) (Bison) ● Ovis canadensis auduboni Merriam (Mountain Sheep) *= introduced specie

Early Records of Some Mammals from Nebraska

Ten species of Recent mammals that formerly occurred in Nebraska have been extirpated there by European man. Surprisingly few specimens of the species concerned are preserved in museum collections. Perusal of a representative segment of the historical literature pertaining to Nebraska has revealed records of occurrence of most of the extirpated species that provide a basis for estimating the former distribution of each in the State

Distribution and Taxonomy of Mammals of Nebraska

Contents Introduction Environment Climate Soils Geology and Physiography Vegetation Effects of Man on The Envmonment Factors Influencing Distribution And Speciation Wisconsin Glaciation And Post-Wisconsin Climates Emigration of Recent Mammals To Nebraska Some Effects of The Environment The Missouri River as a Barrier to Dispersal Geographic Varlatlon and Speciation Mammalian Distributional Areas Summary Treatment and Acknowledgments Checklist (with page references) of Mammals of Nebraska Accounts of Species Introduced Mammals Species of Unverified occurrence Type Localities Literature Cited ● Order MARSUPIALIA-Marsupials ● ● Family DIDELPHIDAE-Opossums ● Didelphis marsupialis virginiana Kerr (Opossum) ● Order INSECTIVORA-Insectivores ● Family SORICIDAE-Shrews ● Sorex cinereus haydeni Baird (Masked Shrew) ● Blarina brevicauda brevicauda (Say) (Short-tailed Shrew) ● Blarina brevicauda carolinensis (Bachman) (Short-tailed Shrew) ● Cryptotis paroa parva (Say) (Least Shrew) ● Family TALPIDAE-Moles ● Scalopus aquaticus caryi Jackson (Eastern Mole) ● Scalopus aquaticus machrinoides Jackson (Eastern Mole) ● Order CHIROPTERA-Bats ● Family VESPERTILIONIDAE-Vespertilionid Bats ● Myotis evotis evotis (H Allen) (Long-eared Myotis) ● Myotis keenii septentnonalis (Trouessart) (Keen\u27s Myotis) ● Myotis lucifugus canssima Thomas (Little Brown Myotis) ● Myotis lucifugus lucifugus (Le Conte) (Little Brown Myotis) ● Myotis subulatus stlbulatus (Say) (Small-footed Myotis) ● Myotis volans interior Miller (Long-legged Myotis) ● Lasionycteris noctivagans (Le Conte) (Silver-haired Bat) , ● Pipistrellus subflavus subf/avus (F Cuvier) (Eastern Pipistrelle) ● Eptesicus fuscus fuscus (Palisot de Beauvois) (Big Brown Bat) ● Eptesicus fuscus pallidus Young (Big Brown Bat) ● LasiUTUS borealis borealis (M tiller) (Red Bat) ● LasionycUTUS cinereus cinereus (Palisot de Beauvois) (Hoary Bat) ● Nycticeius humeralis humeralis (Rafinesque) (Evening Bat) ● Family MOLossIDAE-Free-tailed Bats ● Tadarida brasiliensis mexicana (Saussure) (Brazilian Free-tailed Bat) ● Order LAGOMORPHA-Hares, Rabbits and Pikas ● Family LEPORIDAE-Hares and Rabbits ● °Oryctolagus cuniculus (Linnaeus) (European Rabbit) ● SylviZagus audubonii baileyi (Merriam) (Desert Cottontail) ● Sylvilagus /loridanus mearnsii (J A Allen) (Eastern Cottontail) ● Sylvilagus floridanus similis Nelson (Eastern Cottontail) ● Lepus califomicus meZanotis Mearns (Black-tailed Jackrabbit) ● Lepus townsendii campanius Hollister (White-tailed Jackrabbit) ● Order RODENTIA-Rodents ● Family SCIURIDAE-Squirrels and Allies ● Tamias striatus griseus Mearns (Eastern Chipmunk) ● Eutamias minimus pallidus A Allen) (Least Chipmunk) ● Marmota monax bunkeri Black (Woodchuck) ● Spermophilus franklinii (Sabine) (Franklin\u27s Ground Squirrel) ● Spermophilus richardsonii elegans Kennicott ( Richardson\u27s Ground Squirrel) ● Spermophilus spilosoma obsoletus Kennicott (Spotted Ground Squirrel) ● Spermophilus tridecemlineatus pallidus J A Allen (Thirteen-lined Ground Squirrel) ● Spermophilus tridecemlineatus tridecemlineatus (Mitchill) (Thirteenlined ● Ground Squirrel) ● Cynomys ludovicianus ludovicianus (Ord) (Black-tailed Prairie Dog) ● Sciurus carolinensis pennsylvanicus Ord (Gray Squirrel) ● Sciurus niger TU/iventer :E: Geoffroy St-Hilaire (Fox Squirrel) ● Glaucomys volans volans (Linnaeus) (Southern Flying Squirrel) ● Family GEOMYIDAE-Pocket Gophers ● Thomomys talpoides cheyennensis Swenk (Northern Pocket Gopher) ● Thomomys talpoides pierreicolus Swenk (Northern Pocket Gopher) ● Geomys bursarius lutescens Merriam (Plains Pocket Gopher) ● Geomys bursarius majusculus Swenk (Plains Pocket Gopher) ● Family HETEROMYIDAE-Pocket Mice and Kangaroo Rats ● Perognathus fasciatus fasciatus Wied-Neuwied (Olive-backed Pocket Mouse) ● Perognathus fasciatus olivaceogriseus Swenk (Olive-backed Pocket Mouse) ● Perognathus flavescens flavescens Merriam (Plains Pocket Mouse) ● Perognathus flavescens pemiger Osgood (Plains Pocket Mouse) ● Perognathus flavus piperi Goldman (Silky Pocket Mouse) ● Perognathus hispidus paradoxus Merrlam (Hispid Pocket Mouse) ● Perognathus hispidus spilotus Merriam (Hispid Pocket Mouse) ● Dipodomys ordii luteolus (Goldman) (Ord\u27s Kangaroo Rat) ● Dipodomys ordii richardsoni A Allen) (Ord\u27s Kangaroo Rat) ● Family CASTORIDAE-Beavers ● Castor canadensis missouriensis V Bailey (Beaver) ● Family CRIcETIDAE-Native Rats and Mice ● Reithrodontomys megalotis dychei J A Allen (Western Harvest ● Mouse) ● Reithrodontomys montanus albescens Cary (Plains Harvest Mouse) ● Reithrodontomys montanus griseus V Bailey (Plains Harvest Mouse) ● Peromyscus leucopu,s aridulus Osgood (White-f~t~d Mouse) ● Peromyscus leucopus noveboracensis (Fischer) (White-footed Mouse) ● Peromyscus maniculatus bairdii (Hoy and Kennicott) (Deer Mouse) ● Peromyscus maniculatus luteus Osgood (Deer Mouse) ● PerOmYscus maniculatus nebrascensis (Coues) (Deer Mouse) ● Onychomys leucogaster arcticeps Rhoads (Northern Grasshopper Mouse) ● Onychomys leucogaster breviauritus Hollister (Northern Grasshopper Mouse) ● Sigmodon hispidus texianus (Audubon and Bachman) (Hispid Cotton Rat) ● Neotoma cinerea rupicola J A Allen (Bushy-tailed Woodrat) ● Neotoma floridana baileyi Merriam (Eastern Woodrat) ● Neotoma floridana campestris J A Allen (Eastern Woodrat) ● Synaptomys cooperi gossii (Coues) (Southern-Bog Lemming) ● Synaptomys cooperi relictus Jones (Southern Bog Lemming) ● Jrficrotus ochrogaster haydenii (Baird) (Prairie Vole) ● Microtus ochrogaster ochrogaster (Wagner) (Prairie Vole) ● Microtus pennsylvanicus finitus Anderson (Meadow Vole) ● Microtus pennsylvanicus pennsylvanicus (Ord) (Meadow Vole) ● Microtus pinetorum nemoralis V Bailey (Woodland Vole) ● Ondatra zibethicus cinnamominus (Hollister) (Muskrat) ● *Family MURIDAE-Old World Rats and Mice ● *Mus musculus Linnaeus (House Mouse) ● *Rattus floroegicus (Berkenhout) (Norway Rat) ● Family ZAPODIDAE-Jumping Mice ● Zapus hudsonius pallidus Cockrum and Baker (Meadow Jumping Mouse) ● *Family CAPROMYIDAE-Nutria and Allies ● *Myocastor coypus (Molina) (Nutria or Coypu) ● Family ERETlDZoNTIDAE-Porcupines ● Erethizon dorsatum bruneri Swenk (Porcupine) ● Order CARNIVORA-Carnivores ● Family CANIDAE-Coyote, Wolves and Foxes ● Canis latrans latrans Say (Coyote) ● Canis lupus nubilus Say (Gray Wolf) ● Vulpes velox velox (Say) (Swift Fox) ● Vulpes wIpes regalis Merriam (Red Fox) ● Urof!Yon cinereoargenteus ocythous Bangs (Gray Fox) ● Family URSIDAE-Bears ● Ursus americanus americanus Pallas (Black Bear) ● Ursus horribilis horribilis Ord (Grizzly Bear) ● Family PROCyoNIDAE-Raccoon and Allies ● Procyon lotor hirtus Nelson and Goldman (Raccoon) ● Family MusTELIDAE-Weasels, Skunks and Allies ● Mustela frenata longicauda Bonaparte (Long-tailed Weasel) ● Mustela frenata primulina Jackson (Long-tailed Weasel) ● Mustela frenata spadix (Bangs) (Long-tailed Weasel) ● Mustela nigripes (Audubon and Bachman) (Black-footed Ferret) ● Mustela nivalis cainpeStris Jackson (Least Weasel) ● M ustela vison letifera Hollister (Mink) ● Gulo luscus luscus (Linnaeus) (Wolverine) ● Taxidea taxus taxus (Schreber) (Badger) ● Spilogale putorius interrupta (Rafinesque) (Spotted Skunk) ● Mephitis mephitis hudsonica Richardson (Striped Skunk) ● Lutra canadensis interior Swenk (Otter) ● Family FELIDAE-Cats ● Felis concolor hippolestes Merriam (Mountain Lion) ● Lynx canadensis canadensis Kerr (Lynx) ● Lynx rufus rufus (Schreber) (Bobcat) ● Order ARTIODACTYLA-Even-toed Ungulates ● Family CERvIDAE-Wapiti, Deer and Allies ● Cerous canadensis canadensis Erxleben (Wapiti) ● Odocoileus hemionus hemionus (Rafinesque) (Mule Deer) ● Odocoileus virginianus macrourus (Rafinesque) (White-tailed Deer) ● *Dama dama (Linnaeus) (Fallow Deer) ● Family ANnLocAPRIDAE-Pronghorn ● Antilocapra americana americana (Ord) (Pronghorn) ● Family BovIDAE-Bovids ● Bison bison bison (Linnaeus) (Bison) ● Ovis canadensis auduboni Merriam (Mountain Sheep) *= introduced specie

Kanamycin resistance during in vitro development of pollen from transgenic tomato plants

Effects of kanamycin on pollen germination and tube growth of pollen from non-transformed plants and from transgenic tomato plants containing a chimaeric kanamycin resistance gene were determined. Germination of pollen was not affected by the addition of kanamycin to the medium in both genotypes. Kanamycin, however, severely affected tube growth of pollen from non-transformed plants, while pollen from plants containing the chimaeric gene were less sensitive and produced significantly longer tubes at kanamycin concentrations between 200-400 mg l-1. Apparently, this resistance for kanamycin correlates with the expression of the chimaeric gene during male gametophytic development.

Polarized interacting exciton gas in quantum wells and bulk semiconductors

We develop a theory to calculate exciton binding energies of both two- and three-dimensional spin polarized exciton gases within a mean field approach. Our method allows the analysis of recent experiments showing the importance of the polarization and intensity of the excitation light on the exciton luminescence of GaAs quantum wells. We study the breaking of the spin degeneracy observed at high exciton density $(5 \ \ 10^{10} cm ^2)$. Energy level splitting betwen spin +1 and spin -1 is shown to be due to many-body inter-excitonic exchange while the spin relaxation time is controlled by intra-exciton exchange.Comment: Revtex, 4 figures sent by fax upon request by e-mai