COMPLEX NEURAL ARCS. 93 



same experimental conditions must be used for a large variety of sub- 

 jects with very different natural adaptability, the demands on the sub- 

 ject must be definite and simple. Disturbing influences must be reduced 

 where they can not be eliminated. Conditions must be as natural as 

 possible. (5) Finally, since we aimed to concentrate apparatus and tech- 

 nique so that the different experiments should follow with minimum loss 

 of time and a minimum change in the position of the subject, elaborate 

 or bulky apparatus was inexpedient. 



The instrument which was devised to meet these conditions was not 

 an accident. Back of it are some years of effort to produce the perfect 

 exposure of a word without eye-movements, which accurately dupli- 

 cates a normal fixation in reading. The instrument is in no sense a 

 tachistoscope. It makes no pretense to satisfy all the desiderata of 

 a perfect exposure apparatus. It does satisfy our particular experi- 

 mental needs without serious defects. One new principle involved in its 

 construction will doubtless be of general use, namely, the pendulum 

 stop. It is a device to stop rapid movements of an object suddenly, 

 but with as little noise and as little vibration as is possible. 



In type, our exposure apparatus is characterized by a rapid move- 

 ment of the visual field like the Erdmann-Dodge 1 gravity tachistoscope 

 or the commercial models of Ranschburg, Wirth, and Rupp memory 

 apparatus, or the disk instrument of Dodge. 2 The principle of this 

 type of apparatus is to place the fixation marks and the stimulus word 

 on the same surface, which sudden^ and rapidly moves, at the moment 

 of stimulation, to replace the insignificant fixation mark by the signifi- 

 cant stimulus object in the field of clear vision. Since a word is entirely 

 illegible to the motionless eye while it is in rapid motion across the 

 field of vision, the exposure is simultaneous in all parts when the move- 

 ment ceases. Complete adaptation to distance and light is preserved 

 from prestimulation to stimulation period, by identity of background 

 and identity of the plane of the fixation mark and the exposed word. 

 The moment of exposure is the moment of stopping. 



The great difficulty in constructing apparatus of this type has 

 always been to effect the sudden stop without undue noise or disturb- 

 ing vibration of the exposed word. To meet this difficulty is the func- 

 tion of our new device, the pendulum stop. Most of the stops in com- 

 mon use involve considerable noise. If the stop is padded to prevent 

 noise, it is practically sure to produce a rebound or vibration, with con- 

 sequent blurring of the exposed word during the first moment of expo- 

 sure. If the end-movement is damped by oil or air cushion, the moment 

 of exposure is apt to become uncertain. The pendulum stop obviates 

 or minimizes all these sources of disturbance. 



1 Erdmann and Dodge, Psychologische Untersuchungen iiber daa Lesen, Halle, 1898. 

 2 Dodge, An Experimental Study of Visual Fixation. Monograph Supp. of the Psychol. Review, 

 No. 35, 1907. 



