52 ROBERT M. YERKES AND CHESTER E. KELLOGG 



structed what may be termed the camera lucida apparatus. 

 This has been in use for several months in the Psychological 

 laboratory of the Johns Hopkins University and is reported by 

 Professor Watson to work satisfactorily. 



In our search for simple, inexpensive, reasonably convenient 

 and adaptable means of obtaining the desired data of reaction, 

 we have happened upon the four methods or devices which are 

 now to be described. Figure 1 is a diagrammatic representation 

 of these several devices. Since they are not of precisely the 

 same value, we shall point out the chief merits of each in des- 

 cribing them. 



It was our aim to project, in some convenient manner, an image 

 of the maze and of the reacting subject upon a record sheet 

 which should bear a diagram of the maze. Upon this record 

 sheet the experimenter may accurately trace the path of the ani- 

 mal, while, at the same time, keeping a record of the time of 

 reaction. From the graphic record of the route taken by the 

 animal, the distance and error data may be read. 



We shall designate the four methods as the direct method 

 (Fig. 1, I); the simple reflection method (Fig. 1, II); the double 

 reflection method of Watson (Fig. 1, III) ; and the double reflection 

 method of Kellogg (Fig. 1, IV). 



I. The direct method. This is the simplest device which we 

 have been able to imagine. Above the maze, Z, of Fig. 1, I, is 

 placed either a simple or a compound lens, L, and at the proper 

 distance above it, a plate of glass, D, conveniently framed in a 

 drawing table, and enclosed by a hood, H. Upon this plate of 

 glass, a thin sheet of paper bearing a plan of the maze is placed. 

 As the observer looks down upon the record sheet, he sees an 

 image of the maze and of the reacting subject, and at will he may 

 trace with pen or pencil upon the record sheet the path followed 

 by the subject. 



This method has the important advantages of being extremely 

 simple, inexpensive, and adaptable. It gives a reversed image 

 of the maze, but this is no considerable disadvantage. The 

 chief disadvantages of the method are its inconvenience in 

 connection with large mazes because of the great distance nec- 

 essary between maze and drawing board. But even with very 

 large mazes, the method may be used to advantage if a vertical 

 distance of twenty to thirty feet is available. This arrangement 



