224 //. N. Davis — Method of Demonstrating 



Art. XXIII. — On a Method of Demonstrating Newtorfs 

 Colors by Transmitted Light ; by H. N. Davis. 



It is well known that if white light be passed through a thin 

 film, part of it will be reflected twice within the film and will 

 cause interference and color phenomena. These are usually 

 very faint because the amount of light that is thus reflected is 

 so small as compared with what passes directly through, as to 

 have but a slight effect. If, however, the same wave-front be 

 passed through a uniform series of films, successive portions of 

 certain colors should be blotted out in each film, while other 

 colors which get through the first film without interference, 

 should emerge from each of the other (similar) films without 

 interference, and the color effect should be cumulative, 



At the suggestion of Professor Barus, these surmises have 

 been empirically verified, and excellent results obtained. If a 

 number of wire rings of the same size be mounted in parallel 

 planes, and dipped together into a soap solution, their planes 

 being kept perpendicular to its surface, a suitable series of 

 films results, through which light can be passed and caught on 

 a sheet of paper, showing the desired phenomena very beauti- 

 fully. Since each film, under the action of gravity, is a very 

 thin wedge, the colors are in horizontal bands, appearing first 

 at the top (where the wedge is thinnest) and moving slowly 

 down across the field as the films evaporate, to be succeeded 

 by other bands of lower orders. Indeed good films will often 

 hold until two-thirds of the field is colored with the yellowish- 

 brown of the first order. And if the paper be replaced by a 

 good lens, and the colors projected on a large scale upon a 

 suitable screen, they can be strikingly demonstrated to a class. 

 Some of the effects obtained in this way are most magnificent, 

 even rivaling the best of our autumn sunsets, until it is only 

 with reluctance that one concedes the essential dissimilarity 

 between the two phenomena. In practice, the important thing 

 seems to be uniformity in size and alignment in the set of 

 rings. I have found it convenient to make them some 5*5 cm in 

 diameter, using galvanized iron wire (d=l'2 mm ) and forming 

 each around a pattern of wood or metal, the ends being twisted 

 together into a sort of handle. Such rings can be temporarily 

 strung on three rods, notched at appropriate intervals to insure 

 parallelism in the planes of the rings, while the "handles" are 

 being clamped between two pieces of soft wood. The rings 

 should be at least a centimeter apart to avoid cylindrical and 

 irregular films, and from fifteen to thirty are sufficient. A tin 



