Gravitational lensing of point sources located inside the lens caustic is known to produce four images in a conguration closely related to the source position. We study this relation in the particular case of a sample of quadruply-imaged quasars observed by the Hubble Space Telescope. Strong correlations between the parameters dening the image conguration are revealed. The relation between the image conguration and the source position is studied. Some simple features of the selected data sample are exposed and commented upon. In particular, evidence is found for the selected sample to be biased in favor of large magnication systems. While having no direct impact on practical analyses of specic systems, our results have pedagogical value and deepen our understanding of the mechanism of gravitational lensing.

Abstract
1. INTRODUCTION
2. THE DATA SAMPLE
    2.1. Normalised Coordinates
    2.2. Correlations
    2.3. Two Parameters to Define a Quad Configuration
3. SIMPLE LENS MODEL
    3.1. The Model
    3.2. Model Predictions: General Features
    3.3. Locating the Quasar Inside the Caustic from theImage Configuration
    3.4. The Lens Location: Comparison with ModelPrediction
4. SUMMARY
REFERENCES

• P. Tuan-Anh(Department of Astrophysics, Vietnam National Space Center, VAST)
• T. T. Thai(Department of Astrophysics, Vietnam National Space Center, VAST)
• N. A. Tuan(University of Science and Technology of Hanoi (USTH), VAST)
• P. Darriulat(Department of Astrophysics, Vietnam National Space Center, VAST)
• P. N. Diep(Department of Astrophysics, Vietnam National Space Center, VAST)
• D. T. Hoai(Department of Astrophysics, Vietnam National Space Center, VAST)
• N. B. Ngoc(Department of Astrophysics, Vietnam National Space Center, VAST)
• P. T. Nhung(Department of Astrophysics, Vietnam National Space Center, VAST)
• N. T. Phuong(Department of Astrophysics, Vietnam National Space Center, VAST)