PATTERN-DISCRIMINATION IN VERTEBRATES 



331 



The first set of gratings used for this work were mounted by 

 Cobb. 11 The mounting in the present instrument is essentially 

 that of his ; in fact some of his patterns were used in making the 

 present castings. I have made two modifications to adapt the 

 instrument to the present work. One of these is the device 

 which permits an instantaneous shift between two given widths 



FIG. 3. Mounting for gratings. 



of visible striae; the other is the device which permits instan- 

 taneous and measurable change in direction of the striae. The 

 following description and accompanying figures are of the 

 apparatus in its present form. 



requirement of an ' ideal ' test-object, which is that the contrast between the dark 

 and bright portions of the field should be sharp. Due to the construction of the 

 bright striae, they appear ' fuzzy ' at all values. For the present work this is not 

 seriously objectionable. For some work it might be. 



11 Cobb, P. W., " The Influence of the Illumination of the Eye on Visual Acuity," 

 American Journal of Physiology, 1911, pp. 76 ff. 



