228 Pennington — A Chemico-Physiological 



red and green, as well as blue, while if too concentrated 

 nearly all the light is cut out. 



For accurate work, then, such conditions will not suffice. 

 Hence it became necessary to devise a means by which such 

 defects as are above mentioned should be remedied. After 

 many trials the method about to be outlined was adopted 

 as most satisfactory. 



Description of Color Screens. 



White earthenware bowls, having a diameter of 9^ 

 inches at the top, $% inches at the bottom, and 4^ inches 

 in heisfht, served to contain the material to be investi- 

 gated. Into these bowls were fitted dishes of white glass, 

 two inches in height, and having straight sides. Into 

 these the desired color solution was poured. Tin- foil was 

 fitted over the edge of the glass dish and made to extend 

 some distance below the top of the bowl, all white light 

 being in this way excluded. The dish was covered by a 

 glass plate to prevent evaporation and the entrance of dust. 

 It is advisable to mark the original level of the liquid by a 

 diamond scratch, and see that this level is maintained by 

 adding the proper solvent from time to time. If two 

 layers of the colored liquids were necessary, a second glass 

 dish was placed on top of the first, and the joint made tight 

 as before. 



By such a contrivance, all the light reaching the interior 

 of the bowl must pass through the entire depth of colored 

 medium, since the side light is excluded by tin foil. The 

 light intensity is increased by the reflection from the white 

 walls of the bowl. 



The colored solutions were tested by a No. 7 Kriiss 

 spectroscope, and the limits of the colored band determined 

 as closely as possible. Monochromatic light is, in the strict 

 sense of the term, the light obtained from a single wave 

 length. But it is quite obvious that such light is not prac- 

 ticable for physiological experiments. Hence the term 

 "monochromatic light" has been adopted for a compara- 



