Author Description

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1911 Excerpt:...is this portion of the lighting which will be most extensively used, and where a poor installation would do the most harm. It has already been pointed out that the proper illumination on the reading surface should vary from 2 to 5 feet candles, the exact intensity to be determined by the nature of work to be done at each particular detail. For work, such as examining detail drawings, as high an intensity as 4 or 5 feet candles would be necessary, while for casual observance of large photographs, or the reading of large type would require but 2 or 3 feet candles. Before taking up the means of producing these results, it might be well to consider for a moment the effect of side illumination. It has been found by means of experiments that a person who is able to read type of a given size under local lighting where the source is well shaded from the eyes, is unable to distinguish the same type if a bright source of light is placed or temporarily placed at one side. It is found that the visual acuity is inversely proportional to the angle made by the source of side illumination, and the plane of the reading table, and that any light source within an angle of 26 degrees tends to decrease the visual acuity to a marked extent. Everyone has experienced annoyance due to seeing a light source out of the corner of one's eye when reading, which only proves the truth of our conclusion. A familiar proof of this fact has probably been experienced by all motorists driving at night. Ap proaching an arc lamp one is able to distinguish details in the road very plainly due to the brilliant illumination of the head lights, but after passing through the intense light due to an outside arc, it is impossible to distinguish a single detail until the eyes have recovered from exposure...