METHODS OF STUDYING VISION IN ANIMALS 



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beams lie very near together, as e. g., red and orange, the prisms 

 Wj and m, are unnecessary. Beams from regions lying very near 

 together, e. g., two in the red, can still be spaced and reversed 

 within the limits of the construction of the double slit (cf. p.62). 



At times it becomes necessary to have two beams of exactly 

 the same wave-length, for example, when one desires to test 

 the difference limen for any given color (D. L.). The apparatus 

 permits this with only a slight modification (not shown in the 

 above diagram, but separately below — fig. 6A). 



One allows a single beam to issue from the double slit and to 

 fall upon a Wollaston double image prism, combined with two 

 parallelopipeds of glass, the arrangement of which is show^n 

 in the figure. A, slit admitting the monochromatic bundle, B; 



Figure 6A — Apparatus for obtaining two beams of same wave-length. 



C, Wollaston double image prism; D, one of the parallelopipeds 

 of optical glass (ecjuivalent to two total reflection prisms) ; E, 

 one of the divisions of the bundle, B. This compound prism 

 (designed especially for our work by Steeg & Reuter, Homburg, 

 V. d. Hohe) yields us two parallel beams of light, 10 mm. be- 

 tween centers, of exactly the same wave-length and of equal 

 energy. They can be spaced and reversed as are the red and 

 green in fig. 6. 



As may be seen from the solid line marked G, fig. 6, the green 

 beam falls upon the face of the total reflection prism, wZj, passes 

 through the small achromatic lens ^t,, strikes the total reflection 

 prism, 37, w^hence it is reflected at right angles to fall upon the 

 speculum mirror, L. It is reflected downward at right angles 

 by this and made to fall upon a plaster of paris strip below, 



