CH. XV] DARK GROUND ILLUMINATION 651 



the center of this opening. If, now, light is slightly deviated it will 

 not get through the small opening in the diaphragm and the 

 inhomogeneity will appear as a dark shadow on a light background. 



EXPERIMENTS ILLUSTRATING NORMAL VISION AND SIMPLE, 

 REFRACTIVE EYE DEFECTS 



916. Apparatus needed for the demonstrations: 



Suitable room for projection; White screen 70 to 100 centi- 

 meters (28 to 40 inches) square; Arc lamp and magic lantern con- 

 denser with lamp-house, fig. 378-379; Optical bench with a range 

 beyond the condenser of at least 40 cm. (16 in.) ; fig. 159, 378-379; 

 Lantern-slide carrier and lens support, fig. 159, 378-381; Metal 

 holder for four trial lenses, fig. 380; Oculists' trial lenses as shown 

 in fig. 382; Discs of tin or sheet -iron the size of trial lenses, and 

 with holes for the pupil and a stenopasic slit, fig. 399, A, B.; 

 Lantern slides (4) for illustrating accommodation, astigmatism 

 and anisometropia, myopia, etc., fig. 383, 391-392, 401, ( 9i6ab). 



916a. The cost of the special apparatus needed for the demonstrations 

 in normal and defective vision: 



Metal lens holder for 4 trial lenses $2.25 



Trial lenses in trial rings (14 at 20 cts.) 2.80 



Double trial lenses for unlike eyes i .50 



Lantern slides (4 at 35 cts.) i .40 



$7-95 



To this amount should be added the cost of the object and lens blocks and 

 the vertical pieces for carrying the lens holder and the slide-carrier. 



The screen of white cardboard and the lengthening rods for the optical 

 bench are also extra, but all of these should not make a total outlay of over 

 $10.00 in addition to the magic lantern. As will be seen in the appendix, magic 

 lanterns cost all the way from $20 to $500. 



The trial lenses, lens holder and double lenses may be obtained through a 

 local optician, or they can be got direct from a manufacturer of spectacles, etc., 

 for example: Aloe & Co., St. Louis, Mo.; Bausch & Lomb Optical Co., 

 Rochester, N. Y., New York City, Washington, D. C., Chicago, 111., San 

 Francisco, Cal.; Geneva Optical Co., Geneva, N. Y., and Chicago, 111.; 

 Hardy & Co., New York City, Chicago, 111., Denver, Col., Atlanta, Ga., 

 Dallas, Tex.; Lloyd & Co., Boston, Mass.; E. B. Meyrowitz, New York City, 

 Minneapolis and St. Paul, Minn.; Williams Brown & Earle, and Joseph 

 Zentmayer, Philadelphia, Pa., and many others. 



916b. The authors feel greatly indebted to Dr. Melvin Dresbach and to 

 Dr. Albert C. Durand for suggestions and criticism in the preparation of the 

 manuscript for these experiments in normal and defective vision. 



