VISION IN THE MOUSE 557 



Conclusions: (From the experiments under problem, 1, A.) 

 That the mouse discriminates between light and dark objects 

 under indirect illumination is evident from experiments (a) and 



(t>). 



Experiment (c) shows that both object and background are 

 influential in determining the reaction. 



The albino mouse was influenced by the environment more than 

 the brown mouse. This is shown in the case of the white mouse D 

 making 100 per cent right choices, i.e., choices of white background 

 when the food boxes were uncovered. This result is quite in har- 

 mony with the biological theory of protective coloration. 



B. Under direct illumination 



Apparatus : In fig. 3 is shown a view of the apparatus used in 

 these experiments. It consists of two parts, an experiment box 

 (32 cm. x 52 cm.) and a light box (32 cm. x 98 cm.) Between 

 these two parts is a slide carrying ray filters. A is a nest box 

 (29^ cm. x 18 cm. inside) from which the animal can enter the 

 compartment B through a door /. From B (20 cm. x 17 cm.) it 

 cannot pass back into A directly, but must enter one of the smaller 

 compartments in front, which open into alleys on each side. 

 From one of these alleys the animal reaches the nest box by a 

 gate 0. The two small compartments, (each 14^ cm. x 8 cm.) 

 which may be entered from B, are illuminated by the light from 

 electric lamps in the light box, which enters the compartments at 

 G and R through two apertures each 6| cm. square. These aper- 

 tures, in the experiments now to be described, were covered with 

 ground glass. In the light box the lamps can be moved back and 

 forth to give the required differences in intensity, their distance 

 from the ground glass being measured on a scale S. The light 

 box is divided lengthwise by a partition which insures the illumi- 

 nation of each aperture by the appropriate lamp only. The slide 

 carries three rectangular cells (15x16 x 6 cm.) separated from one 

 another by pieces of felt. -These, filled with colored media, can 

 be used as ray filters for tests of color discrimination. During the 

 present experiments they remained empty. 



