STUDIES IN SPECIAL SENSE PHYSIOLOGY 361 



Hering's Case of Total Colour-blindness. 



Tests were also carried out by means of an ingenious polari- 

 scopic method. At one end of a horizontal tube, blackened inside, 

 a cork plate was fixed ; the plate was perforated in the middle 

 and a doubly refracting prism inserted. The other end of the 

 tube was closed by a lid in which two equal and symmetrically 

 placed semicircular openings were made. With this contrivance 

 the ordinary and extraordinary images obtained by polarisation 

 appeared to form a series of spherical surfaces when the tube was 

 directed to a source of light e.g. a piece of baryta paper stretched 

 over a glass plate. Between the eye and the prism a small tele- 

 scope and a Nicol prism were introduced, together with a graduated 

 arc. The diaphragm of the telescope removed the lateral images, 

 leaving only two magnified white circles, the halves of which could 

 have their brightness altered in opposite directions by rotating 

 the Nicol. In front of one opening in the tube a coloured glass 

 was placed, and the Nicol so arranged that for the normal " dark " 

 or for the totally colour-blind eye, both halves appeared equally 

 bright. The next table gives the readings for two observers. Some 

 of the variations may be explained by the fact that the normal- 

 sighted person was unpractised in this sort of work. 



x For methods of measuring white " valency" consult Hering ( lf ), p. 567, &c. 

 For the present puri>ose the figures in the third column of the table may be regarded 

 as a recalculation of the amounts of white in the sectors which match the coloured 

 circles, so as to admit of comparison with the figures of the fourth column. 



