METHODS OF STUDYING VISION IN ANIMALS 37 



Spectroscopic, as well as spectro-photometric tests would 

 have to be made frequently. But in spite of this, it seems to 

 us desirable that a standard set of gelatine filters be manu- 

 factured. Such a set of films would possess almost all of the 

 merits of colored papers, while enabling the experimenter to 

 vary, somewhat, the intensity of his chromatic stimulus inde- 

 pendently of its hue and saturation. 



Available at present are the following sets of pigmented 

 gelatine films : 



Hanauer vereinigten Gelatoid Fabriken, Hanau, a. M. Ger- 

 many, manufactures colored gelatine films, some of which, 

 singly or in combination, are excellent. They may be obtained 

 through the American Agent of the German firm, Henry Pfaltz, 

 300 Pearl Street, New York. 



Queen ^nd Company, 121 1 Arch Street, Philadelphia, im- 

 ports and sells a set of thirty gelatine absor|Dtion films, 3^- by 

 6^ inches, stained by chemically pure substances. They are 

 too thin for our purposes, and in our tests proved of little value. 



Zimmermann, 21 Emilienstrasse, Leipzig, Germany, supplies 

 gelatine films for psychological purposes. 



Filters are made, to order, by the Cramer Dry Plate Company, 

 St. Louis, Missouri. 



Dr. Louis Bell recommends the following formulae, on the 

 basis of his experience: 

 For red: (i) Grubler's lichtgrun F. S. ; (2) concentrated safra- 



nine, in gelatine films. 



This double filter transmits pure red of 700 t^i^ wave-length. 

 For yellow: (i) Copper chloride (solution) in absolute ethyl- 

 alcohol; (2) yellowish eosine (Berlin Anilinf abrik) , in gela- 

 tine film. 



This combination, solution and gelatine, filter transmits yel- 

 low of 560-590 /*/^. 

 For green: (i) Acid green (Cassella and Co.); (2) methyl orange 



III (St. Denis Co.), in gelatine films. 



This double filter transmits green of 460-490 /^/^ (?). 

 For blue green: ]\Iethyl green, in gelatine film. 



This transmits light of 450-490 /«•/*. 

 For blue: (i) Alkali blue (Albany Color Works); (2) yellowish 



eosine (Berlin Anilinfabrik) , in gelatine films. 



This double filter transmits from 435 z^/* to the ultra-violet. 



