58 20BERT M. YERKES AND JOHN B. WATSON 



saturation. The saturation boxes are employed when it is 

 desired for purposes of control, to alter the saturation of the beam. 



The above diagram shows the three saturation boxes in the 

 dark cabinet in the position in which they are to be used. A 

 separate drawing of the interior of the saturation box is shown 

 below the cabinet. The boxes are first cast in brass; the walls 

 are then shaped down until they are made as thin as possible, 

 especially at the large end, which is practically knife-edged. 

 This end is made rectangular, 7 mm. x 7 cm. A piece of opal 

 glass, first ground with coarse emery upon the " flashed " sur- 

 face (see p. 26) is then accurately ground by hand to fit the 

 whole opening snugly (see dotted lines in small figure). It 

 is then forced in (with flashed surface out) after the edges have 

 been carefully blackened with dead black paint and allowed 

 to dry. This gives a sharply defined surface for the beam com- 

 ing from the speculum mirror to fall upon. On account of the 

 shape of the box the excess light falls into the dark cabinet 

 and is there absorbed. 



The interior construction of this saturation box is clear from 

 the drawing. A small block of wood-fiber, F, is fitted into the 

 small end. A miniature lamp-socket is imbedded in this block, 

 and the wires for its electrical connections are led outside to 

 two binding posts, VB, BB; a small miniature tungsten lamj) 

 i^" c.p., 2 volts, is burned in this socket. This lamp should 

 have practically a straight filament and should be burned con- 

 stantly in one position. The tip of the bulb should be at the 

 side and not at the end. The three lamps required are supplied 

 with current by five 2 volt rtorage batteries. A separate sliding 

 resistance is burned in series with each lamp. A volt meter 

 reading 0-6 volts should also be placed in the circuit, in such 

 a way that the voltage of each lamp may be tested separately. 

 The lamp is burned always at a constant and known voltage. 

 Immediately above the lamp a small shelf -support, SH, 2 mm. 

 wide and 1-64'' thick, is shown; shelves of smoked glass are 

 accurately ground and fitted into the opening. The intensity 

 of the light can be easily and accurately controlled by this 

 means. At a greater expense an Aubert diaphragm, adjustable 

 from the outside of the box, may be inserted in place of the 

 smoked glass. 



The opal glass, S3, is illuminated from below by this white 



