22 ROBERT M. YERKES 



For most animals there is no necessity of locking the doors of 

 the apparatus, but when it is to be used with monkeys or anth- 

 ropoid apes, it is absolutely necessary that the experimenter be 

 able to securely lock any one or all of the sliding doors. It is 

 therefore essential to equip the large sized apparatus with locks 

 to be operated in connection with the mechanisms which raise 

 and lower the doors. Each door should lock automatically when 

 lowered and unlock when the raising mechanism is operated. 



Just behind and a trifle above the release box, an observer's 

 stand or record table should be constructed, separated by a 

 screen from the apparatus so that the animal shall not be able 

 to see the observer. On this table there should be placed a 

 keyboard, or lever device, by means of which any one of the 

 twenty-six working doors 4 of the apparatus may be raised or 

 lowered quickly and quietly. 



For the small apparatus the various doors may be controlled 

 readily by means of a light cord, which runs from a screw eye 

 in the top of each door, through appropriately placed pulleys, 

 to a hinged lever key which the observer operates. This key 

 should be so arranged that when it stands in approximately 

 vertical position the entrance door is closed. When it is placed 

 in the horizontal position, the entrance door is open. A cord 

 from the exit door, carried similarly by pulleys, should be so 

 placed that it may be attached readily by means of hook and 

 ring, or ball and slot, to this key, so that if, when a given en- 

 trance door is lowered, the experimenter desires to raise, simul- 

 taneously, the exit door of the same box, the pushing of the 

 key to the vertical position will effect the appropriate move- 

 ment of each door, that is, will simultaneously lower the given 

 entrance door and raise the given exit door. The distance to 

 which the entrance door is raised may be altered by changing 

 the point of attachment of the cord to the key. This simple 

 hinged key and cord device renders necessary the use of only 

 fourteen keys for the operating of twenty-six doors, but the 

 scheme is feasible only so long as the doors in question are light 

 enough to be readily moved by means of a fairly small lever 

 key. The accompanying diagram, figure 5, indicates the rela- 

 tions of parts, as described above. 



4 If both return alleys are used there are twenty-seven doors instead of twenty- 

 six to operate. 



