76 ROBERT M. YERKES AND JOHN B. WATSON 



The metal surface should be warmed quite thoroughly and 

 then spread quickly and evenl}^ with the shellac. The prism 

 should be warmed also and then firmly pressed down upon the 

 metal surface. After drying (for about 2 hours) a satisfactory 

 union is obtained. 



d. Episcotisters; and motor iris diaphragm. — An episcotister 

 supplied \vith movable sectors has its advantages and its disad- 

 vantages. Its chief advantage consists in the fact that it may 

 be set so as to cause only a small loss of energy in the beam. 

 It is made with a rim of metal graduated in degrees, which is 

 attached to the rotating shaft by two fixed sectors of 21° each. 

 The rest of the sectors are free to move concentrically on the 

 shaft, where they can be clamped by a lock nut in any desired 

 position. They can thus be made to close the whole 360° open- 

 ing, or folded in behind the two fixed sectors in such a way 

 that the angular opening reaches its maximum of 318°. They 

 can be arranged thus to give about 88 per cent, of the full in- 

 tensity of the beam. 



Its chief disadvantage consists in the fact that there are so 

 many sectors that an accurate adjustment of the angular open- 

 ing cannot be made. In making threshold tests or tests of the 

 DL for intensity and in accurately adjusting energy relations 

 of any kind by the use of the selenium cell, the episcotister 

 supplied with movable sectors is out of the question. It is, 

 however, extremely valuable for qualitative work , where one 

 desires quickly to present a color stimulus first with high, then 

 with intermediate and finally with low intensity. Zimmerman, 

 through the Arthur H. Thomas Company, Philadelphia, fur- 

 nishes three sizes of adjustable episcotisters — 10 cm. opening, 

 38 marks; 15 cm. opening, 50 marks; 20 cm. opening, 65 marks. 

 The first two sizes are especially adapted to our work. 



In order to make a satisfactory sector for use in all quan- 

 titative experiments one uses the frame work, shaft, pulleys, 

 etc., of the one just described, but unscrews the rim with its 

 two fixed sectors and takes off the free sectors. Two sectors 

 are then prepared in the following way: Two discs of brass 

 10 cm. in diameter — or 15 cm., depending on the size of the 

 Zimmerman episcotister — are drilled at the center to fit the 

 shaft of the episcotister. The one of these discs taking the 

 place of the rim and the two fixed sectors of the above, should 



