Volume 1

Opus majus / edited, with introduction and analytical table by John Henry Bridges.

  • Roger Bacon
Date:
1897-1900]
Catalogue details

Licence: In copyright

Credit: Opus majus / edited, with introduction and analytical table by John Henry Bridges. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

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    VOL. II. PAGES The rays, in this case, coming from the threads of the web are so nearly coincident with the rays coming from the object, as to be con- fused with them ........... 104 106 CHAPTER Iil. Of Perception through reasoning. Many examples can be given of this. The most striking is our perception of distance. An object may be so distant, as to subtend so small an angle in the eye that vision ceases. Short of that limit, the degree of distance is deter- mined by a continuous series of objects between the object and the eye. In a flat country we have no means of judging the height of the clouds, which we can do when we see them on the sides or summits of mountains. [Clouds would appear to be of no great height, though, as we know from the fact of twilight, exhalations other than clouds may rise fifty-one miles. Such exhalations are not aqueous: being dry they retain the sun’s heat better, and thus rise higher.] Forjudging, then, of distance, we must have an inter- mediate series of objects, each of which shall be appreciable by the eye with sufficient accuracy. These limits are soon exceeded. A line of trees appears continuous, though there may be a great interval between each of them. So planets seem to be in the same surface as fixed stars, though the difference of remoteness is immense. So an equilateral figure of many sides becomes at a distance undistinguishable from a circle. A circle may be taken for a straight line, a sphere for a plane figure. When a circle is held sideways before the eye, the part nearer to the eye will be recognized as nearer if the distance is moderate : if it is very far off, the difference of distances in the points bears so small a proportion to the whole as not to be recognized. Thus it is that, when the moon is in her first or third quarter, the circular line defining the light part from the dark appears as a straight line. So too the sun and moon seem to us flat, though they are spherical 106-108 CHAPTER IV. All this is exemplified in the study of the Moon’s Phases. The base of the cone of solar light occupying the moon’s surface appears to us twice in the month as a straight line; otherwise as curved : a fact unexplained in the Latin translations of Aristotle and Averroes. Here we have to leave the region of sense, and penetrate to the real facts, which, but for the remoteness of the sun and moon, we should be able to see. The boundary of rays proceeding from one eye to the moon is a great circle of that body; that of rays proceed- ing from the sun to the moon is also a great circle, or nearly so. At conjunction and at full moon these circles coincide, the lunar surface