284 



SCIENCE 



[N. S. Vol. XLIII. No. 1104 



■uniformly striated field in the telescope. This 

 is emphatically the case for the parallel rays, 

 h' and a'; but with the crossed rays a and 6 



n n 



71 % 71 



^^. 



(X* 



Fig. 4. 



the interference is confined to the rays in the 

 equidistant positions, n, in Fig. 4, and mid- 

 way between them the field is a neutral yellow. 

 In other words between the rays n, the rays 

 are displaced laterally as shown by the arrows 

 (recalling the arrangement of nodes in acous- 

 tics), so that corresponding rays a and ol for 

 instance, do not coincide and hence can not 

 interfere, the region aa! (Fig. 4) remaining 

 neutral. In Fig. 5, the rays crossing at a 

 vanishing angle have been shown for three ray 

 filaments and the transverse arrows indicate 

 the directions in which the rays have been 

 urged, laterally. ITaturally I am merely 

 stating the case as suggested by the results. 



One may argue that there may be a secondary 

 periodicity in the grating. But why does it 

 not appear at all in the case of parallel pencils, 

 when it is so obtrusive in the case of crossed 

 pencils of rays? Again the interferences are 

 imquestionably due to D^ and B^ light, simul- 

 taneously. If the grids in these two cases 

 should be at a slightly different angle to each 

 other, their superposition would give some- 

 thing like the observed phenomenon apart from 

 details. With white light the linear phenom- 

 enon would eventually become achromatic. 

 But why should lines so close together as T)^ 

 and jD, show any appreciable difference of 



angle in their interference pattern? Inter- 

 secting interference grids, moreover, can be 

 produced by other methods and always betray 

 their origin. The final inference is that sug- 

 gested by Figs. 4 and 5, that homogeneous rays 

 on crossing (here in a medium of plate glass) 

 may exert a lateral influence on each other, 

 to the effect that identical rays emerging from 

 the crossing are arranged in equidistant nodal 

 planes according to Fig. 4. 



Carl Baeus 

 Brown University, 

 Providence, R. I. 



ANNUAL MEETING OF THE CHICAGO 

 ACADEMY OF SCIENCES 



On the evening of Tuesday, January 11, the 

 annual meeting of the Chicago Academy of Sci- 

 ences was held at the Academy building ia Lin- 

 coln Park, Chicago. 



The guest of honor and chief speaker was Di- 

 rector Frederic A. Lucas, of the American Mu- 

 seum of Natural History, and his address was en- 

 titled "The Service of the Museum to the Pub- 

 lic." 



The usual reports were received from the offi- 

 cers of the academy, and the results of the annual 

 election were read. The ofl&cers for the ensuing 

 year are: President, John M. Coulter; First Vice- 

 president, Henry Crew; Second Vice-president, 

 Stuart Weller; Secretary, Wallace W. At wood; 

 Treasurer, Henry S. Henschen. 



Following the business meeting the members 

 and guests were invited to inspect the new ex- 

 hibit, which extends through the central portion 

 of the main museum floor. It consists of one 

 large case, 75 feet long and 20 feet wide. In this 

 case, and supported from the ceiling, fifty-six of 

 the larger birds of the Chicago region were in- 

 stalled as in flight. The exhibit is viewed from 

 the main floor, and is 8 to 10 feet above the level 

 of the eye, so that the birds are seen much as they 

 might be under fortunate circumstances out-of- 

 doors. One hundred and four habitat groups have 

 now been installed in the Academy Museum to il- 

 lustrate the natural history of the Chicago region. 

 The birds, flowers, insects, reptiles and mammals 

 are represented, and with the completion of this 

 plan the museum will be unique in America, and 

 have a special educational effectiveness. 



Announcement was also made of the following 

 course of open lectures: 



