50 ROBERT M. YERKES AND JOHN B. WATSON 



as is shown in the small auxiliary diagram. We thus have 

 falling upon the plaster of paris surface a sharply focussed and 

 magnified image of slit G. This image is always larger than 

 the plaster of paris strip. The excess light passes down into a 

 dark compartment and is absorbed. 



The red beam, after passing through the small right angle 

 prisms in the way already described, escapes behind the base 

 of the prism tn^, passes through the small projection lens 34, 

 thence to the speculum plate M, and down to the plaster of 

 paris surface, as in the case of the green. As conditions are 

 in the diagram, the red beam is on the left, the green on the 

 right. 



In order to reverse the position of the two colors, the slide N, 

 bearing the prisms m^ and m^ must be pushed over to the 

 right to take the dotted position. The green beam now engages 

 the right angle prism m^, is bent at right angles through lens 

 32 to meet prism 38, thence to a mirror and plaster of paris 

 surface as before. After this change is made, the red beam 

 is made to pass between m^ and m^. Its course is in no way 

 changed by the shifting of the slide N; as conditions are now 

 after the introduction of this change, the green beam is on the 

 left and the red on the right. If only t'WO speculum mirrors 

 are used, the carrier P, holding them must be pushed over to 

 the left to take the dotted position. If three speciHum plates 

 are used, no shifting of them is required (cf. p. 56). It is hardly 

 necessary to mention that m^ and m^ are prisms of the same size 

 and absorption value; that prisms 37 and 38 are likewise matched, 

 and that lenses 32 and 33 are matched as to quality of glass, 

 thickness, and focal length. 



There are three convenient ways in which the energy of these 

 beams may be altered: (a) by attaching iris diaphragms to the 

 lenses; (b) by the interposition of a smoked wedge in the path- 

 way of each beam, and (c) by the rotating sector or episcotister. 

 The rotating sector is the easiest and most accurate method of 

 the three. Figs. 7 and 11 show the arrangement for use with 

 the episcotister cf. also p. 76). 



The method by which a white beam of light equal in size 

 and form to the monochromatic beam is made to fall upon the 

 plaster of paris strip in place of either of the monochromatic 

 beams is as follows : A Nernst filament is mounted vertically 



