=84. Kinds of standards.= The base standard is the one taken at Lincoln (latitude 41° N.) at meridian June 20–22. This is properly the unit to which all exposures are referred, but it has been found convenient to employ the Minnehaha standard as the base for the Colorado mountains, in order to avoid reducing each time. _Relative_ standards are frequently used for temporary purposes. Thus, in comparing the light intensities of a series of formations, one to five standards are exposed on the solio strip before beginning the series of readings. _Proof_ standards are the exposed solio strips, which fade in the light, and can, in consequence, be kept only a few weeks without possibility of error. The fading can be prevented by “toning” the strip, but in this event the exposures must be fixed in like manner before they can be compared. This process is inconvenient and time-consuming. It is also open to considerable error, as the time of treatment, strength of solution, etc., must be exactly equivalent in all instances. _Permanent_ standards are accurate water-color copies of the originals obtained by the photometer. These have the apparent disadvantage of requiring a double comparison or matching, but after a little practice it is possible to reproduce the solio tints so that the copy is practically indistinguishable from the original. The most satisfactory method is to make a long stroke of color on a pure white paper, since a broad wash is not quite homogeneous, and then to reject such parts of the stroke as do not match exactly. Permanent standards fade after a few month’s use, and must be replaced by parts of the original stroke. _Single_ standards are made by one exposure, while _multiple_ ones have a series of exposures filling a whole light strip. These are regularly obtained by making the exposures from 1–10 seconds respectively, and then increasing the length of each successive exposure by 2 seconds. Single exposures of 1–5 seconds as desired usually serve as the basis for permanent standards, but a multiple standard may also be copied in permanent form. Exposures for securing standards must be made only under the most favorable conditions, and the length in seconds must be exact. The use of the stop-watch is imperative, except where access may be had to an astronomical clock with a large second hand, which is even more satisfactory. The length of time necessary for the series desired is reckoned beforehand, and the exposures begun so that the meridian falls in the middle of the process.