July 2, 1909] 



SCIENCE 



29 



nests," for we regard the nesting instinct as 

 no more inexplicable than the other instinct- 

 ive actions which recur with almost clock-like 

 regularity in the reproductive cycle of most 

 birds. The entire round of activities which 

 leads to the production of a certain type of 

 nest, as in the robin, vireo, or oriole, is without 

 doubt remarkably uniform and stable. But 

 it is far less stable or uniform than the con- 

 ditions which determine the form and color of 

 the egg. If this is true it is not altogether 

 surprising to find some open nests with snow- 

 white eggs, and some closed ones, like the mag- 

 pies', in which the eggs are spotted. Tet no 

 one could maintain that the behavior of the 

 wild bird is to be explained by any simple 

 formulae, at any point. 



The abundant illustrations which have been 

 drawn from a variety of sources, are naturally 

 uneven in proportion and quality, half-tones 

 of photographs from life having been excluded 

 to keep down the weight, but the plan thus 

 followed has certainly led to variety in abun- 

 dant measure. Francis H. Herrick 



SPECIAL ARTICLES 



A SIMPLE FABRY AND PEROT INTERFEROMETER 



During a course of experiments with inter- 

 ferometers it was found that a very simple 

 and inexpensive Fabry and Perot instrument 

 could be constructed of plate glass which gives 

 results almost as good as the costly inter- 

 ferometer. The construction of this appa- 

 ratus for demonstration purposes will well 

 repay the teacher and student. The sharp- 

 colored interference rings obtained by using 

 luminous gases in vacuum tubes as sources are 

 extremely beautiful. The D lines from a 

 sodium burner are easily separable. If the 

 interference pattern using a copper or iron 

 arc is focused on a wide slit of a single prism 

 spectrometer, a section of the interference 

 rings is seen in the various spectrum lines, 

 illustrating the method of Fabry and Buisson 

 for the determination of the new standard 

 table of wave-lengths. The Zeeman effect 

 can also be easily shown with this apparatus. 



Take two pieces of plate glass about an inch 

 square (I have used the so-called German 



plate) and silver" them till one surface of 

 each plate cuts down the intensity of the 

 transmitted light to about a quarter of the 

 incident light. Separate these silvered sur- 

 faces by two strips of cardboard. A useful 

 thickness to begin with is the cover of the 

 24 two-cent postage-stamp book, as this will 

 clearly separate the D lines. Mount these 

 plates over a half-inch hole in a metal plate 

 by means of three pressure screws, two of 

 which are shown in the following diagram, 

 being a section through the center. The third 

 screw is midway between the other two and at 

 the end of the plates. 



Looking normally through the plates at the 

 glowing filament of an incandescent lamp, a 

 number of images of it will probably at first 

 be seen. Adjust the pressure screws until 

 these images are in juxtaposition in the line 

 of sight; the silvered surfaces are then ap- 

 proximately parallel. Place the instrument in 

 a clamp stand, and focus the light from a 

 sodium flame or a vacuum tube upon the 

 plates and look at the interference bands with 

 a small laboratory telescope focused for in- 

 finity. Usually the eyepiece has too large a 

 magnification for the above retardation and 

 it is better to use in place of it a single lens 



of focal length about two inches. At first 

 only a small section of the interference pat- 

 tern is seen, but with a little careful adjust- 

 ment of the pressure screws the whole ring 

 system is obtained in sharp focus. Removing 

 the telescope and with the above lens used as 

 eyepiece, focus the interference system from 

 the above sources or an arc, upon the slit of 

 a spectroscope. The bands in the different 

 spectrum lines are then observed with the 

 telescope on the spectrometer. 



For further suggestions regarding the ad- 

 justments and other experiments for which 

 this apparatus can be used, refer to an article 



^ For silvering solutions see the appendix to 

 Baly's " Spectroscopy." 



