70 ROBERT M. YERKES AND JOHN B. WATSON 



working with a fixed and known difference in energy between 

 the two beams. Where quantitative relations are to be thus 

 maintained, special sectors, described on p. 76 must be used, 

 (c) The sectors can be made to interchange with respect to 

 the two beams (turn sectors as before to position B, fig. 12, 

 but do not lock cord at 87, fig. 7). This is desirable where 

 abrupt changes in brightness are to be introduced. For this 

 work the Zimmerman sectors are best. One of the sectors can 

 be set with a wide angular opening, the other with a small opening. 

 The intensity of the two beams is thus automatically changed 

 with every change in the left-right position of the beams. Care 

 must be taken in making this setting that both of the colors 

 remain clearly supra-liminal for the animal when the sector 

 with small angular opening respectively engages them. 



4. Description and cost of parts of apparatus 

 a. Sources and their control. — Sunlight and heliostats. A 

 good heliostat greatly increases the ease with which sunlight 

 may be used. There is no cheap and satisfactory heliostat on 

 the market. Nearly all of them are supplied with clockwork 

 which is too weak to drive them steadily. They are usually 

 made too light to withstand gusts of wind. Furthermore the 

 mirrors attached to them are usually silvered on the back. 

 This enormously decreases the intensity of the reflected beam 

 and gives rise to a series of disturbing secondary images. The 

 mirrors may be silvered on the anterior surfaces, but the silver 

 film deteriorates rapidly. 



The Fuess heliostat is a convenient one. It permits of use 

 in any latitude and will reflect the beam in any direction. It 

 is listed by Max Kohl (supplied through the Arthur H. Thomas 

 Company) at $125. At the Johns Hopkins University Professor 

 Anderson has modified the oj-iginal castings of Professor Row- 

 land, and has devised a massive heliostat supplied with two 

 heavy speculum mirrors. These mirrors are ground and polished 

 by Brashear to a surface equal to that of good plate glass, which 

 is accurate enough for the requirements of most work on vision 

 done in psychological laboratories. This heliostat can be sup- 

 plied for about $150. 



Arc lamp. — The automatic arc of Bausch and Lomb with 

 adjustable rheostat of 25 amperes capacity is especially recom- 



