The straight lines in this diagram represent divisions of time; the ver tical ones showing the month and LIcly, and LUC 111J11Z011Lil1 ones the time of day as shown by a sun-dial.
The curved lines are curves of equal altitudes of the sun, computed for the latitude of Washington, for the year 1889.
The combination of the two sys tems of lines is designed to enable the photographer, whether amateur or professional, who has at some time determined the length of exposure required under certain circumstances of subject, clouds, lens, diaphragm, plate or film, etc., to decide what ex posure to give under the same cir cumstances, at any time between sunrise and sunset, on any day of the year.
The diagram is based upon one constructed for the latitude of Lon don, published in the Photographic News, in 1887, and reprinted in the ANNUAL of 1888, and the same stand ard of comparison is used in this adaptation—that is, such circumstan ces of subject, clouds, lens, diaph ragm, plate or film, etc., as will require an exposure of one second, at noon of any day between the 4th of April and the 7th of September, or at any time between a quarter to ten in the forenoon and a quarter past two in the afternoon on the 21st of June.
The diagram, although constructed for the year 1889, and for the lati tude of Washington, will serve equal ly well for any other year, and well pnniicrb for nrrlinary nnrnncec throughout the United States (ex clusive of Alaska), although in the extreme Northern and Southern belts it will not be accurate.
The diagram is strictly accurate for " apparent time " only, but it is sufficiently so for " local mean time " (which may differ sixteen minutes from " apparent time "), and in the great majority of places for "standard time" (which in some places differs half an hour from "mean time," and may differ three-quarters of an hour from " apparent time)." FABRIC.—Photographic fabric is a name generally given to a translucent cloth dyed of a golden yellow or red color, and used for dark room windows. It is always advisable to use two thicknesses, as there are usually a number of fine holes, almost invisible, but which, at the same time, allow of the entrance of white light.