6 'Journal of Comparative Neurology and Psychology. 



long periods of time without allowing sensible alteration in its 

 intensity. Since the arc is not in the dark-room where the animals 

 react, the noise made by it is not a source of disturbance. When 

 burning well, this arc rarely makes a noise which is noticeable even 

 near at hand, after the heavy wooden box (metal lined) has been 

 closed. 



The lens used as a condenser is an ordinary 4" biconvex read- 

 ing glass with 8" focus. This lens gives a clearly defined image 

 of the crater of the positive carbon at its focus. 



The slit, Si, which is placed at the focus of Li, is a common op- 

 tical slit with knife edge opening. Its width was adjusted once 

 for all to give a spectrum of high intensity and was never thereafter 

 changed. 



The collimating lens, L^, is a heavy 31/^" achromatic lens with 

 a focal distance of 18". The slit, *S'i, is in the focus of this lens, 

 consequently the face of the 65 mm. heavy flint glass prism, P, is 

 filled with parallel light admitted by it. The now refracted beam 

 falls upon the mirror, M^ (the use of this mirror is necessitated 

 by the narrowness of the room) which is silvered upon its anterior 

 surface. This mirror reflects the beam through the achromatic 

 lens, Lg, (similar in all respects to L^ except that Lg's focal distance 

 is 24"). 



The lens, L^, brings the refracted beam to a focus upon the double 

 slit, 82, (not shown in Figs. 1 and 2, but shown separately in 

 Fig. 3) in a series of colored images of 8^^. The solar lines are 

 plainly visible and serve as a guide in the selection of particular 

 wave lengths. The apparatus can be so arranged (by revolving 

 Mj upon its axis) that these lines shall coincide with definite mm. 

 divisions of the slit.^ 



In order that this double slit may be more easily understood, the 

 following description is given: Fig. 3 shows the slit horizontally. 

 The sharply outlined spectrum falls upon the polished surface of 

 the slit between on the 8C scale, and on the 8C^ scale. Two 

 selected portions of the spectrum pass out between the knife edge 



'For example, the Na line can be made to fall upon 3 of the scale >S'C etc. 



