tables to simplify the computations, (4) the 

 device in its present form can be called accu- 

 rate for some purposes, but that the present 

 accuracy can be increased if necessary, and 

 (5) when the above improvements are made, 

 this photographic method will be a valuable aid 

 in the racial analysis work being carried out 

 by the Fish and WildUfe Service for the Inter- 

 national North Pacific Fisheries Commission. 



SUMMARY 



1 . The photographic measuring device 

 described in this paper was developed to aid 

 the work of the International North Pacific Fish- 

 eries Commission in differentiating North 

 Pacific salmon stocks from Asiatic salmon 

 stocks by racial characteristics. The taking of 

 accurate measurements of the salmon is a 

 necessary part of this work. 



are: 



2 . The advantages inherent in photography 



(1) The pictures of the fish would 

 form permanent records from which 

 measurements could be checked and re- 

 checked. 



(2) When fish were needed for meas- 

 urements that were not previously con- 

 templated, these measurements could be 

 taken by the biologist from the many 

 pictures of fish that will have accumu- 

 lated over the years instead of his having 

 to wait until the next fishing season ar- 

 rives and then obtain measurements from 

 only one year's sample. 



(3) In the case of interpretation of 

 visual characteristics, such as net marks, 

 pictures can be analyzed in the laboratory 

 by one or more persons skilled at such 

 jobs. 



3. When two cameras are used in deter- 

 mining the longitudinal distance between two 

 points on a three-dimensional object such as a 

 fish, there are two errors involved: (1) parallax 

 and (2) perspective . Theoretically, both of 

 these errors can be corrected. 



4. The method of obtaining the longitud- 

 inal distance between two points on a three- 

 dimensional object as described in this paper 

 involves the use of the photographic measuring 

 device and the use of a microscope with a cali- 

 brated stage . 



The photographic measuring device con- 

 sists of (1) the base and framework, (2) the 

 strobe light, (3) the power pack, (4) the camera 

 assembly, (5) the automatic features, and (6) 

 the wiring. 



The microscope was slightly modified 

 by taping two glass plates, one on top of the 

 other, upon the calibrated stage of the micro- 

 scope. The glass plates serve to hold the film 

 flat when the film is inserted between them . 



5 . The steps in obtaining a longitudinal 

 distance between two points on a three-dimen- 

 sional object with this photographic method are: 

 (1) Assemble the device, (2) set cameras and 

 connect wiring, (3) align the subject parallel with 

 the orientation line on the base, (4) take the pic- 

 ture, (5) take the necessary film readings, and 

 (6) compute the longitudinal distance between the 

 two points . 



6. The device is not fully portable owing 

 to the fact the the base is not designed to be dis- 

 assembled. 



The degree of efficiency that the automatic 

 features are responsible for has not been deter- 

 mined, but they are considered to be valuable 

 time and labor savers. 



(4) Photography and its associated 

 tools promise to lend themselves very 

 readily to automation, thus tending to 

 minimize human error, which in turn 

 minimizes bias of operators . 



More fish can be handled and recorded 

 on film using this device than the methods now 

 in use, but the photographic method has two ad- 

 ditional steps. That is, reading the film and 

 computing from the readings the longitudinal 

 dimensions of an object such as a fish. Four 

 film measurements are needed to compute one 



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