52 ROBERT M. YERKES AND JOHN B. WATSON 



by the letter D. The wall A is the outer wall of the building- 

 It faces west (south is a far better exposure if it can be obtained). 

 With a Fuess heliostat, sunlight may be obtained from about 

 I p. M. to 4 p. M. 



A shelf, B, lo" wide and 2" thick, supported upon heavy 

 trestle work 20" in height, projects through a window in the 

 wall A. Continuous with this, at an angle of 120 degrees to it 

 and in the' same plane, is a much wider shelf, C. This latter- 

 shelf, about 30" wide and 4' long, is surrounded by walls 2" 

 thick and 12" high. The compartment thus formed is sup- 

 plied with a heavy wooden cover not shown in the diagram. 

 Its shape is somewhat irregular, as appears in the figure. Most 

 of the delicate parts of the apparatus are assembled here. It is 

 lined with dark paper and is made light-tight and dust-proof. 

 Where possible, it is very desirable to have the supports for- 

 the parts of the apparatus thus far described made of concrete. 



Upon the end of shelf B, w^hich projects through the wall A, 

 is to be found a circular aluminum plate, i, to hold the heliostat, 

 when sunlight is desired. Two end supports, 3 and 4, screwed 

 to the shelf, support a steel bar, 2. To this rod is attached an 

 arc mechani::m, 5, and a clamp, 6, which carries the prism table,- 

 7, shown inside box C. Two castings, 8 and 9, carry \" rods, 

 10, II, 12 and 13, which are so geared that the arc may be 

 controhed by the operator in the room 7, by turning the knobs 

 17 and 18. These long rods are supported by means of cast- 

 ings, 14, 15 and 16.^ 



The incident beam (sunlight or arc) is made to pass through 

 the condensing lens 19. This is a "second-hand" Perry portrait 

 lens, 3" aperture, 6" focus, supplied with rack and pinion. 

 It gives an image of the crater of the positive carbon upon slit 



20. The width of the slit is controlled by the micrometer screw 



21. The beam of light may be kept from passing through the 

 slit when desirable by means of a little shutter, 22, electro- 

 magnetically actuated. The shutter can be operated b}^ means 

 of a switch placed near the operator at G. 



After the rays pass through the slit they are made parallel 

 by means of the collimating lens 24 (Voitlander portrait lens 



^ While the ordinary 45° hand feed arc supplied with the controlling rods just 

 described is satisfactory and cheap, the automatic lamp of Bausch et Lomb is much 

 more convenient. When it is used no controlling rods, castings, etc., are neces- 

 sary. (See p. 71.) 



