324 bulletin: museum of comparative zoology. 



successive days on account of deterioration manifested in the eye hy a 

 gradual diminution in the migration, so that usually only one, or 

 sometimes two, legitimate matchings in amounts of migration evoked 

 by the two colors could be obtained for each animal. 



In order that the results might be compared and summarized in 

 some way, it was necessary to assign a quantitative value to the 

 amount of migration exhibited by each recorded sketch after com- 

 paring it with an arbitrary standard. This standard was a circle, 

 made with the same stamp as those in the records, with a dark area 

 in the center representing a medium amount of migration which, 

 based on a scale of 5, was evaluated at 3. Each migration-record, 

 therefore, was referred to this standard, judged for its relative amount 

 of migration, and assigned a value accordingly. Table II contains 

 these evaluations in the vertical columns under the distances at 

 which the exposures were made (50, 100, etc. cm.). In the right hand 

 portion of the table are given those distances — deduced from the 

 assigned values either directly, where one in red equalled one in blue- 

 violtt, or indirectly by interpolation — at which red is probably 

 equivalent to blue- violet in eliciting the migration. Discretion had 

 to be ex.rcised in determining the validity of evaluations chosen for 

 comparison, because, owing to deterioration in the response of the 

 pigment according to the sequence of the exposures, certain cases 

 apparently equal in the table were quite incomparable. Taking 

 animals K and L for example, the exposures for both were in the fol- 

 lowing sequence: — to BV at 400 cm. (Jan. 24), 300 cm. (Jan. 26), 

 200 cm. (Jan. 27); to R at 150 cm. (Jan. 30), 100 cm. (Jan. 31), 

 50 cm. (Feb. 1), the progress through each color being from the lower 

 to the higher intensity. Although two days of rest were allowed 

 between the last exposure to blue-violet and the first to red, yet the 

 exposure on the third consecutive day to red showed evidence, if 

 one compares the values for 50 cm. with those at 100 cm., of exhaustion 

 in the eye. Again, in animal 38, where red was tested at 50 cm. at 

 the beginning and again at the end of the series, this deterioration was 

 manifested. Final deductions from the assigned evaluations were 

 made, therefore, after the sequence of exposures had been taken into 

 consideration. The distances at which the efficiencies of the two col- 

 ors were on a par were averaged and equated, thus R at 107.5 cm. = 

 BV at 247 cm. Since the latter distance was 2.29 times the former, 

 the intensity of the former was (2.29) ^ times the latter, i. e. BV = 

 5.2 R in terms of power to elicit the migration of pigment. 



In order to verify the results obtained by this necessarily artificial 



