number of a plate in the H. & D. system
is found by dividing 34 by a number known as the Inertia, the Inertia,
which is a measure of the insensitiveness of the plate, being determined
according to the directions laid down by Hurter & Driffield--that is, by
using pyro-soda developer and the straight portion only of the density
curve. If, for instance, the Inertia was found to be one-fifth, then the
speed number would be 34 / 1/5 = 170, and the plate is H. & D. 170. The
W.P. No. is found by dividing 50 by the Inertia. Thus 50 / 1/5 = 250, and
the plate is W.P. 250, but for all practical purposes the W.P. No. can be
taken as one and a half times H. & D. The Wynne F. numbers may be found by
multiplying the square root of the Watkins number by 6.4. Thus

[sqrt]250 = 15.81, and 15.81 x 6.4 = W.F. 101.

For those photographers who are in the habit of using an actinometer giving
the plate speeds in H. & D. numbers, the following table, taken from the
_Photographer's Daily Companion_, is given, {106} which shows at a glance
the relative speed numbers for the various systems. The Watkins and Wynne
numbers only hold good, however, when the inertia has been found by the H.
& D. method.

TABLE OF COMPARATIVE SPEED NUMBERS FOR PLATES AND FILMS

------------------------------------------------------
|H. & D.|W.P. No.|W.F. No.||H. & D.|W.P. No.|W.F. No.|
--------+--------+-----------------+--------+---------
| 10 | 15 | 24 || 220 | 323 | 114 |
| 20 | 30 | 28 || 240 | 352 | 120 |
| 40 | 60 | 49 || 260 | 382 | 124 |
| 80 | 120 | 69 || 280 | 412 | 129 |
| 100 | 147 | 77 || 300 | 441 | 134 |
| 120 | 176 | 84 || 320 | 470 | 138 |
| 140 | 206 | 91 || 340 | 500 | 142 |
| 160 | 235 | 103 || 380 | 558 | 150 |
| 200 | 294 | 109 || 400 | 588 | 154 |
------------------------------------------------------

Although theoretically the higher the speed of the film the less the
duration of exposure required, there is a practical limit, as besides the
intensity and actinic value of the light admitted to the film a definite
time is necessary for it to overcome the chemical inertia of the sensitised
coating and produce a useful effect. With every make of film it is possible
to give so short an exposure that although light does fall upon the film it
does no work at al