METHODS OF STUDYING VISION IN ANIMALS 77 



be 1-16" in thickness and the other should be 1-32". Each. of 

 these discs should be laid off and cut out as follows : Two diam- 

 eters are marked out at right angles to each other. Opposite 

 quadrants of the four thus formed should be sawed out, leaving 

 an outside rim of 5 mm. and an inside rim of 10 mm. at the 

 axle. One of the quadrants should be graduated in degrees. 

 This work should be done with great accuracy. The heavier 

 of the two discs is then screwed into the bushing on the axle 

 which formerly held the fixed disc of the episcotister. The 

 thin disc is slipped on in place of all the movable sectors. It 

 is clear that the two opposite open quadrants of the movable 

 sector can be made to coincide either with the open or closed 

 sectors of the fixed disc. In the one case is given the maximum 

 opening of 180°, and in the other the minimum, 0°. The gradu- 

 ation on one of the quadrants shows one-half the total open- 

 ing always. 



When it is remembered that the energy of an interrupted 

 beam of light is directly proportional to the angular opening 

 of the rotating sectors, it is seen that when once the full energy 

 of the beam has been determined by means of the selenium cell, 

 the energy of the interrupted beam becomes known from the 

 angular setting of the episcotister. It is possible thus to make 

 liminal and differential threshold-tests with accuracy. 



In making settings of the episcotister of less than 2° (i. e., 

 of less than a reading of 1° on the scale), a graduated wedge 

 should be used, permitting an opening as small as 1-100° to 

 be made with accuracy. In making threshold tests this opening 

 will have to be obtained by the use of an angular slit mechanism 

 controlled by a micrometer screw. The opening can be set 

 with an accuracy of i-iooo mm. 



Any small high speed motor may be used to drive the epis- 

 cotisters. Motor No. 5700, no D. C, 1-20 H. P. of the C. H. 

 Stoelting Company, Chicago, costing $15, is very satisfactory. 

 The circular starting box and resistance usually supplied with 

 it should not be used, Vjut in its place should be substituted 

 a sliding rheostat of suitable winding (75 ohms). James G. 

 Biddle, Philadelphia, supplies a duty free "G. R." rheostat, 

 type E. H., 75 ohms resistance, of excellent construction, for 

 $5-36. 



