SUNLIGHT AND ITS MEASUREMENT 189 



After deciding what feature of radiation it is desired to meas- 

 ure and what degree of accuracy is necessary, the instrument 

 must be selected with these purposes in mind as well as with 

 an appreciation of the conditions under which the measure- 

 ments are to be performed, in order that both errors of manip- 

 ulation and errors of interpretation may be guarded against. 

 There are many excellent methods for measuring radiation. 

 All have their advantages and disadvantages and simplicity 

 of operation is not the only criterion by which they should be 

 judged. The most sensitive and the most precise instruments 

 have peculiarities that can only be successfully met by suf- 

 ficient experience in their manipulation, so that for only oc- 

 casional use for brief periods they should not be selected. It 

 may be added that the most sensitive instruments do not nec- 

 essarily yield the most accurate results. Such instruments 

 are usually delicate and with careless treatment or in the hands 

 of persons ignorant of their principles of construction they may 

 give results more at variance with the truth than less precise 

 apparatus. It is unfortunately too common in biological 

 literature to find work published that is wholly misleading 

 in' its implications and expressed statements of accuracy be- 

 cause the author refined one portion of the method adopted 

 to a high degree and completely ignored sources of large error 

 in other portions. The growing use of the "probable error" 

 is to be deplored on that account: that observations agree 

 among themselves is no guarantee of the correctness of the 

 results and an account of investigations of the instrumental 

 and experimental errors w T ould be far more convincing and of 

 real value. 



There are three general methods for measuring radiation and 

 each has been developed along several lines so that the subject 

 is complicated both because of its terminology (which, it may 

 be added, is not in all cases either stable or rational) and be- 

 cause each instrument has certain disadvantages and advan- 

 tages depending upon the purpose for which the user intends 

 it. Although it is out of the question to present this extensive 

 subject in any detail, still it seems that a good purpose will 



