How this can be done we have shown in the 17th chapter, and in the first Principle of the 19th chapter.
13. So much of the light. We will now speak of the mode in which objects must be presented to the senses, if the impression is to be distinct. This can be readily understood if we consider the processes of actual vision. If the object is to be clearly seen it is necessary: (1) that it be placed before the eyes; (2) not far off, but at a reasonable distance; (3) not on one side, but straight before the eyes; (4) and so that the front of the object be not turned away from, but directed towards, the observer; (5) that the eyes first take in the object as a whole; (6) and then proceed to distinguish the parts; (7) inspecting these in order from the beginning to the end; (8) that attention be paid to each and every part; (9) until they are all grasped by means of their essential attributes. If these requisites be properly observed, vision takes place successfully; but if one be neglected its success is only partial.
14. For instance, if any one wish to read a letter that has been sent him by a friend, it is necessary: (1) that it be presented to the eyes (for if it be not seen, how can it be read?); (2) that it be placed at a suitable distance from the eyes (for if it be too far off, the words cannot be distinguished); (3) that it be directly in front of the eyes (for if it be on one side, it will be confusedly seen); (4) that it be turned the right way up (for if a letter or a book be presented to the eyes upside down or on its side, it cannot be read); (5) the general characteristics of the letter, such as the address, the writer, and the date must be seen first (for unless these facts be known, the particular items of the letter cannot be properly understood); (6) then the remainder of the letter must be read, that nothing be omitted (otherwise the contents will not all be known, and perhaps the most important point will be missed); (7) it must be read in the right order (if one sentence be read here and another there, the sense will be confused); (8) each sentence must be mastered before the next is com-
