METHODS OF STUDYING VISION IN ANIMALS 87 



on their upper surface. This grill when properly acid blackened 

 reflects no light. At first sight objection may be raised to it 

 for the reason that it slows the progress of the animal. This 

 is a distinct gain rather than otherwise. It is distinctly more 

 difficult for the animal to make a hurried dash to the food 

 boxes. It forces leisure for choice. Since there are no sharp 

 points the animals are not discommoded by it, and rapidly 

 become accustomed to making the proper steps. The cover 

 for the box should be made in a similar way, only in this case 

 the metal strips need not be insulated. Furthermore, the strips 

 may be separated from one another by a distance of 2" or more. 

 If small animals like the rat or mouse are used, a woven wire 

 cover may be placed above the cover just described. After 

 the animal iias had a few days' experience in the box, the remain- 

 ing tests may be made without the use of the cover. 



(c). Ordinary high grade dead black velveteen may be used 

 for lining the walls and partition of the experiment box. The 

 addition of the velvet lining is merely a precautionary measure 

 since the diaphragms effectually pre\'ent all lateral reflection. 



When the experiment box is used with the color apparatus, 

 one other matter should be considered. It will be remembered 

 that the experiment box has to be shifted 25 cm. each time 

 the relative position of the two colors is reversed. It is just 

 possible that such a movement of the experiment box might 

 give the animal a secondary criterion for making a choice. In 

 order completely to eliminate such a possibility it is well to 

 make compartment A, fig. 4, entirely separate from B. Com- 

 partment A, containing the animal, may then be removed and 

 the apparatus adjusted for a new test before A and B are joined. 



VI. CONCLUSIONS AND SUMMARY OF RECOMMENDATIONS 

 The purpose of this investigation has finally turned out to 

 be the devising of apparatus and modes of procedure by which 

 highly accurate determinations may be made (a) of the limen 

 (threshold) light and color stimuli for an animal; (b) of the 

 limits of sensibility with respect to wave-length and intensity; 



(c) of the difference limen for light and for chromatic stimuli; 



(d) of the stimulating values of different chromatic stimuli 

 (curve of luminosity for the eye of a given animal should thus 

 be obtained); (e) of the chief characteristics of adaptation; 

 (f) of visual defects in animals. 



