118 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



in these spectra interference bands, which disappear in any 



given part of the field common to the two spectra when the 

 intensities are equal. The instrument is an ordinary spec- 

 troscope, the two spectra being obtained, the one above the 

 other, by the common device of a reflection prism. Between, 

 now, the collimator and the prism the author places a Fou- 

 cault prism to polarize the beam horizontally, a plate of 

 quartz cut parallel to the axis to produce a considerable dif- 

 ference of path between the ordinary and extraordinary 

 rays, and a Wollaston prism to bring back the light rays 

 into two planes at right angles to each other, and at the 

 same time to double the images of each half of the slit. 

 Four channelled spectra are now visible in the instrument, 

 two of these (one polarized vertically, the other horizontally) 

 overlapping. Consequently the brilliant maxima of the one 

 coincide with the dark maxima of the other ; and when the 

 intensities are equal, the bands completely disappear. The 

 method is simple and satisfactory. 



Riicker has given, in Nature^ an account of some interest- 

 ing experiments with black soap films, i. e., films of soap and 

 water so thin that no light is reflected by them, and they 

 appear black. He has observed that, under whatever condi- 

 tions the black film may have been formed, a remarkable 

 and very rapid change of thickness invariably occurs at the 

 boundary which divides the black from the colored portion 

 of the film. By an exceedingly happy method, the thickness 

 of the film was measured by measuring the resistance'of a 

 known area. The value of the resistance of a black ring one 

 millimeter broad was 1,750,000 ohms, from which the calcu- 

 lated thickness is twelve millionths of a millimeter, or one 

 forty-ninth part of the wave-length of D. Various measure- 

 ments prove this thickness to be approximately uniform. 



Thompson has communicated to the London Physical So- 

 ciety, says Nature, a paper on interference fringes within the 

 Nicol prism. If the " field " of a Nicol be explored by the 

 eye, it will be seen to be bordered on one side by a margin 

 of violet-blue light, and on the other, when the light passes 

 obliquely through the prism, by an orange band, within 

 which lie a series of colored fringes; these latter are very 

 clearly seen with monochromatic light, when a second set 

 within the blue band also appears. The author showed that 



