54 . THE HABITAT 
that a national or international station be ultimately selected for this pur- 
pose, in order that light values taken in different parts of the world may be 
teadily compared. 
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 tem- 
porary purposes. Thus, in comparing the light intensities of a series of 
formations, one to five standards are exposed on the solio strip before be- 
ginning 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 solu- 
tion, 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 increas- 
ing the length of each successive exposure by 2 seconds. Single exposures 
of 1-5 seconds as desired usually serve as the basis for permanent stand- 
ards, but a multiple standard may also be copied in permanent form. Ex- 
posures 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 begtm so that the meridian falls in the middle of the process. 
Single standards are exceedingly convenient in photometer readings, but 
they are open to one objection. In the sunshine it is necessary to make in- 
stant decision upon the accuracy of the match, or the exposure becomes too 
