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 

 readily 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. Relath'e 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 i-io 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 nuiltiple 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 eve;n 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. 



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 



