78 ROBERT M. YERKES AND JOPIN B. WATSON 



Slender circular leather belts, glued together and sandpapered 

 until they are perfectly smooth running, should be used for 

 driving the sectors. Lacking these, rubber bands may be 

 used. The latter work well for a time, but soon deteriorate. 

 It is w^ell to keep a supply of them close at hand for emergency 

 use. 



Iris diaphragms ^ with maximal diameter of 2.5 cm., suitable 

 for mounting upon the small projection lenses shown in fig. 9, 

 may be obtained of Spindler and Hoyer through Arthur H. 

 Thomas Company, Philadelphia, for 5 marks. They are not 

 very accurate at best, and their use is recommended only as 

 an additional means of control of intensity. Had we proof 

 that the interruption of a beam of light by the rotating sector 

 alters the intensity of light in the same ratio for the animals 

 as it does for the human being — i. e., according to the law of 

 mean squares — there w^ould be no necessity for any other form 

 of control of intensity. Lacking the actual proof, how^ever, 

 it seems best to have occasional recourse both to the iris dia- 

 phragm and to the smoked wedge. 



e. Mirrors. — Speculum mirrors are far more satisfactory in 

 this work than any other form. They are expensive, however, 

 and difficult to obtain. The expense is due largely to the diffi- 

 culty in manufacture. A model for the mold must first be 

 made in w^ood, from w^hich a metal casting is made. The 

 casting is then accurately planed. The speculum metal is melted 

 and poured into the mold. A constant stream of illuminating 

 gas is made to enter the mold while the molten mass is being 

 poured in. The gas ignites and consumes all the air present 

 in the mold. The plates are removed from the mold and placed 

 in a bed of burning coke for several hours, in order to anneal 

 them properly. After cooling they are ready for grinding and 

 polishing. In the work we are attempting to do there is no 

 necessity for obtaining a planeity of surface greater than that 

 offered by the best plate glass. 



The three large mirrors t," x 5" called for in this apparatus 

 were cast by Mr. Childs, and ground by John A. Brashear of 

 Pittsburg. The total cost of each mirror is about $15. Max 



^ Since the openings in the double slit wall are in the foci of these lenses it fol- 

 lows that the size of the images falling upon the plaster of paris surfaces is not 

 altered by the use of the diaphragm. 



