To use this table to compare the rapidity of two plates, the sensitometer numbers of which are known, run the eye up the column A till the sensitometer number is reached, and then along the line of figures till it reaches the column of figures under the sensitometer number of the second plate, when the figure there shown will tell at once the difference in rapidity. Example - A plate has been used of ordinary rapidity showing 18 on sensitometer, and it is desired to use a plate of sensitometer No. 22: what will be the reduction in exposure ? Find 22 in column A, and carry the eye along the line of figures opposite to it till it meets the column under 18 in B line - the number 3 will be found; therefore the 22 plate is three times as sensi-tive as the No. 18, and will therefore require one-third of the exposure.
(3) The Subject. This is one of the factors much misunder-stood. Experience soon teaches the novice that a short exposure is required for a white object, more for a dark-coloured; some fail to recognise that darkness in colour or tone is not the same thing as darkness in lighting, and that a white object badly lit may require more exposure than a dark object well lit. We may also consider the subject with regard to colour sensitiveness. If we have to copy, for instance, a yellowish object, and use an ordinary plate which is not sensitive to yellow, we must give a much longer exposure than when using a plate which has been " colour sensitised" for yellow, if we wish to show correctly the difference in the lightness or dark-ness of objects.
As a guide to exposure on various subjects, the following table, given on next page, by Mr. Burton may be of special value. The figures giving exposure must be regarded as relative to the season and also to the speed of the plates rather than absolute, and must be corrected by the table of plate speeds, also by the season and hour table already given. The exposure times as they stand in the table apply fairly well to exposures made about noon on a bright day in March, and with plates ranging from 100 to 120.
(4) The Diaphragm. The influence of the diaphragm or stop on exposure has been rather fully dealt with under the headings Diaphragm (q.v.), also incidentally in some of the