METHODS OF STUDYING VISION IN ANIMALS 75 



order at a very reasonable price. The small lens used as a con- 

 denser with the Nernst filament (p. 47) and the small projection 

 lens referred to on p. 51 were furnished by them for $1 each. 

 The lenses of the system need not cost over $100. 



c. Prisms. — The spectrometer, or refracting, prism recommended 

 may be had of O. L. Petitdidier, Lake Ave. and 53d St., Chi- 

 cago, or of Hans Heele (Arthur H. Thomas Company, Phila- 

 delphia). Heele furnishes a dense flint 60° prism 80 mm. aperture 

 (ref. ind. for 0=1.70) for $60. (Extra dense flint, ref. ind. 

 for D=--i.78 for $68.) Petitdidier furnishes a very satisfactory 

 prism 4" X 4" face for $50, another 3V' x 3V' for $30. He 

 rarely carries these sizes in stock, but will make them to order.' 



This prism should be mounted on a metal table 5" in diameter 

 (see p. 7 fig 7). It should be set in minimum deviation for some 

 clearly marked line easy to obtain in sunlight or arc, e.g., the 

 D line, and never thereafter moved. Care must be taken to 

 level the prism very accurately after it has been installed. For 

 this reason it is best to have the top of the prism table made 

 in two parts after the manner described on p. 62. 



Small total reflection prisms of flint or crown glass mav be 

 purchased of Queen and Company, Philadelphia; O. L. Petit- 

 didier, Lake Avenue and 53d St., Chicago, or Bausch and Lomb, 

 Rochester, N. Y. Queen and Company furnish them most 

 cheaply and with all the accuracy required for the present work. 



Two 9 mm. prisms are needed immediately behind the double 

 slit. These may be had for $1.75 each. Two 19 mm. reversing 

 prisms are required, $2.50 each; and two 25 mm. spacing prisms, 

 $3 each. 



A small compound double image prism, a sketch of w^hich is 

 shown on p. 49 is necessary for use in experiments on the liminal 

 difference for intensity (D. L.). This may be had of Steeg und 

 Reuter through the Arthur H. Thomas Company, Philadelphia. 

 The cost of this prism is about $30. 



All prisms should be cemented permanently to their respective 

 tables. Several cements are on the market, but a most satis- 

 factory cement can be made by taking dry shellac and pouring 

 a small amount of 95% alcohol in with it, warming the mixture 

 over the flame until a homogeneous, fairly thick paste is formed. 



* A prism with high refractive index is very likely to tarnish. Mr. Petitdidier 

 recommends a prism with refractive index not greater than 1.66. 



