6. In the tangential way the passage of rays is 7 folds in length, in comparison to the perpendicular passage. Longer passage means more intense attenuation, and results in better visualization.

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First object. This is an x-ray of what?
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A sheet of paper lying on a film is (apparently) unhindered in its penetration, and subsequently not displayed. An orientation of the leaves of this book (it is Roentgen's report on the new rays) tangential to the beam produced enough material to cause a considerable attenuated of radiation. In this projection, we clearly see "the paper”...

8. A ball of silver paper: this paper is only visualized where it is positioned parallel to the beam.

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What happens there?
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There the radiation is significantly attenuated and this attenuation results in sharp lines.

9. It is quite transparent in the centre, but creates sharp shadows along the edge contours.

Bottle in X-Ray
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How can you explain this "x-ray" with the hypothesis that "every radiograph of an object is the sum of all its slices"?
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We see the slices in the left part of the image. Summarizing the slices, the ventral and dorsal wall is only added twice; the edge contours are added a dozen times or more. Therefore, the edge contours dominate.

Calcified fibrothorax
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Why is it difficult or impossible to observe this pathology in the radiography?
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Calcified fibrothorax on both sides of dorso-medial pleura. - Calcification is not projected tangentially and subsequently overlooked in the standard (posterior-anterior) radiograph. (A tiny calcification on the lateral right side is only detectable though very careful observation).