Mabch 27, 1908] 



SCIENCE 



501 



the hermaphroditic condition since a complete 

 return to the dicecious condition normal to the 

 species results in the first sexual generation. 



It will be of interest to see how closely the 

 further discoveries in the mosses follow the 

 conditions worked out in such an unrelated 

 form as Phycomyces. 



A. F. Blakeslee 



Connecticut Ageicultueal College, 

 Stokes, Conn. 



a study of visual fixation 

 Professor Eaymond Dodge begins a series 

 of " Studies from the Psychological Labora- 

 tory of Wesleyan University " with " An Ex- 

 perimental Study of Visual Fixation." The 

 Psychological Review prints the number as 

 its Monograph Supplement for November, 

 1907. 



Professor Dodge finds that the involuntary 

 movements of the eyes during supposed fixa- 

 tion are, in part, compensatory to head and 

 body movements; in part, however, they are 

 " normal and physical disturbances " of fixa- 

 tion, due to irregular head and body move- 

 ments for which compensation is inadequate. 

 The compensatory eye movements are united 

 with the movements of head and body " into 

 a thoroughly organized motor system," fur- 

 nishing a coordinating mechanism capable of 

 " explaining the intimate correspondence be- 

 tween tactual and visual space." Visual mo- 

 tives for the fixation movements are found in 

 retinal fatigue and in the correction of inade- 

 quate binocular coordination. 



In studying control of fixation movements. 

 Professor Dodge remeasured the ocular reac- 

 tion time, using Professor Holt's method with 

 the alternating arc light, the fall of the ex- 

 posure screen giving the stimulus simultane- 

 ously with the actinic beam, the latter being 

 reflected from the cornea to the falling plate 

 of the camera. The alternations of the cur- 

 rent thus gave a time record in a series of 

 dots. Interruptions by a tuning-fork would 

 give greater accuracy. Plates of pot blue 

 glass " stopped down " the arc light to a com- 

 fortable glow without materially reducing the 

 effect on the camera plate. The slow reac- 

 tion time determined by earlier experimenters 



was confirmed, but a minimum of 130 <j was 

 reached. The head reaction was quicker than 

 that of the eye. 



Successive fixations of the same word were 

 found to vary widely in location of the point 

 of regard, but were called " perfect fixations " 

 when the object of interest was " brought to a 

 retinal area of clear vision." The functional 

 center of the retina is larger or smaller accord- 

 ing to the character of the object and accord- 

 ing to the corresponding extent of the area of 

 clear vision. 



Fixation is called " adequate " when it is 

 sufficiently long and accurate to condition a 

 " cleared-up " perception of the object of re- 

 gard. Renewing his criticism of the short ex- 

 posure-times of Zeitler and Messmer, Professor 

 Dodge measures the exposure-time needed for 

 the " clearing up " of words presented upon 

 various pre- and post-exposure fields, and con- 

 cludes that " the shortest adequate fixation 

 pauses in reading are between 70 a and 100 a." 



In further experiments he measured the 

 effect of peripheral factors, such as words seen 

 peripherally in reading, in modifying the total 

 consciousness without properly " clearing up." 

 The movement is from general to special 

 effects, phrase, sentence and paragraph, epi- 

 sode and plot, forming " a dynamic back- 

 ground" for each new word-complex as it 

 clears up. 



Professor Dodge raises the question whether 

 the spatial relations of the total visual field 

 are determined by its relations to the fovea, 

 or whether the object of regard is not " rather 

 determined in its spatial relations by its ap- 

 parent position in the total visual field"; and 

 after criticism of the theory of retinal local 

 signs, he proposes a substitute theory of 

 " genetic organization of the retinal elements." 

 While the argument here is not wholly satisfy- 

 ing, the new facts that are being brought for- 

 ward in this field make it certain that a better 

 theory is to shake out eventually, and Pro- 

 fessor Dodge's view is worth a careful reading. 



The appendix to the article reviews the 

 technique of recording the eye-movements by 

 photographic registration, and describes the 

 apparatus used in the Wesleyan experiments. 



