METHODS OF STUDYING VISION IN ANIMALS 39 



Spectra of hundreds of solutions have been described.' We 

 ha^•e ourselves examined many substances in our search for 

 single solutions which should yield satisfactory monochromatic 

 stimuli, but our efforts have not added materially to the 

 knowledge of the subject. 



Single solutions which transmit "monochromatic light" in 

 fair intensity are rare. Indeed, a serious fault common to filters 

 is the great diminution of the intensity of light. Except with 

 a high intensity source it is impossible, in most instances, to 

 obtain a narrow band of light of fair intensity. This defect 

 is gr, atly accentuated when two or more layers of solutions 

 (or glasses or gelatine films') are used side by side. Another serious 

 defect of ray filters is the transmission of the infra red and 

 ultra violet. 



Wet filters have the advantage of being relati\'ely easy to 

 make. They have almost all of the advantages of dry filters, 

 and their deterioration is a less serious matter because of the 

 ease with which they can be renewed. 



In order to insure "keeping" for a considerable interval, 

 solution filters should be made up with distilled water which 

 has been boiled for several minutes before dye is added. The 

 addition of a few crystals of carbolic acid to solution after it 

 has been cooled will prevent the gro^'^i;h of moulds. 



For single solution filters, boxes of crystal plate, optical glass, 

 or quartz, in certain standard sizes and shapes (or to order) 

 may be obtained from E. Leybold's Nachfolger, ColnaRh., 

 Germany. These cells are admirably suited to the needs of 

 students of color vision who wish to obtain chromatic stimuli 

 by means of wet filters. 



For double or triple solution filters the Leybold glass boxes 

 may be obtained on special order, with a single plate of glass 

 between adjacent chambers. 



We have designed a special unit cell which has many ad- 



' Kayser, H. Handbuch der Spectroscopie. Leipzig, 1905, Bd. III. 



L'hler, H. S. and Wood, R. W. Atlas of absorption spectra. Washington, 

 1907. Carnegie Institution Publication, No. 71. 



Jones, H. C. Hydrates in aqueous solution. Washington, 1907. Carnegie 

 Institution Publications, No. 60. 



Formanek, J. Die qualitative spectral analyse anorganischer Korper. Berlin, 

 1900. 



Formanek, J. Spectral analytischer Nachweis kunstlicher organischer Far- 

 benstoffe. Berlin, 1900. 



