May 6, 1898.] 



SCIENCE. 



615 



vermicelli soup. The eggs and spermato- 

 zoa are discharged and nothing is left but 

 empty skins scarcely visible. 



Professor Michelson described bis im- 

 portant invention of a spectroscope with- 

 out prisms or gratings made by building up 

 steps of equal thickness of optical glass. 

 With twenty elements 5 mm. thick the re- 

 solving power would be 100,000 which is 

 about that of the best gratings. The 

 method is especially important for the ex- 

 amination of single lines and the study of 

 the effects of broadening, shifting or doub- 

 ling of lines. Dr. Gill read a biographical 

 memoir of Edward D. Cope, based on his 

 address as President of the American Asso- 

 ciation, which has been published in this 

 Journal. President Mendenhall gave the 

 results of further researches on the lengths of 

 words used by different authors. He is able 

 to show graphically a characteristic curve 

 for a writer, and thus has found a method by 

 which disputed authorship may be tested. 



Dr. J. S. Billings resigned the office of 

 Treasurer on account of his removal from 

 "Washington, and Mr. Charles D. Walcott 

 was elected in his place. Messrs. Billings, 

 Bowditch, Brush, Hague, Marsh and ISTew- 

 comb were re-elected additional members 

 of the Council for another year. 



No new members of the Academy were 

 elected this year. This appears to be un- 

 fortunate, as only thirteen elections have 

 been made during the past eight years, 

 whereas the Academy has lost twenty- eight 

 members by death. The Academy can, 

 by its constitution, only elect five members 

 annually, and as the deaths are likely to 

 amount to nearly this number it is difficult 

 to see how the membership can be main- 

 tained if, in certain years, no members are 

 elected, as was the case in 1891, 1893, 1894 

 and this year. 



A large addition was, however, made to 

 the foreign associates of the Academy, whose 

 number is limited to fifty, as follows : 



Professor Henri PoincariJ, Paris ; Dr. David Gill, 

 Cape Town ; Lord Eayleigh, London ; Professor 

 Adolf von Baeyer, Munich ; Lord Lister, London ; 

 Professor Edward Suess, Vienna ; Professor H. de 

 Lacaze-Duthiers, Paris ; Professor Edward Strasbur- 

 ger, Bonn ; Professor Felix Klein, Gottiugen ; Profes- 

 sor Henri Moissan, Paris ; Professor Karl Alfred von 

 Zittel, Munich. 



The autumn meeting of the Academy 

 will be held at ISTew Haven, beginning on 

 November 15th. 



803ie aids to the study of stereoscopic . 

 vision: 



The familiar form which the stereoscope 

 has assumed since Brewster, together with 

 the marked development of photography, 

 has brought about a general appreciation 

 of the striking and frequently beautiful 

 effect which this instrument produces. 

 This form of the apparatus, however con- 

 venient, is not best suited to the exposition 

 of the underlying principles of the stereo- 

 scopic illusion. These principles involve 

 the general problem of the perception of 

 depth or solidity, and this, in turn, is a 

 rather complicated matter, which involves 

 many details. An important service 

 which the stereoscope performs for the 

 psychologist is the aid which it renders 

 him in the analysis of these factors. Some 

 of the more or less recent variations in 

 the form and construction of stereoscopic 

 instruments furnish added facilities for the 

 demonstration of the factors which enter 

 into the perception of depth. To furnish a 

 brief account of these various aids to the 

 study of stereoscopic vision is the purpose 

 of this article. 



One of the most frequently discussed 

 points is the dependence of the ap- 

 pearance of solidity upon the dissimi- 

 larity of the two stereoscopic pictures, 

 which, in turn, imitate the differences of 

 the retinal images in the two eyes. The 

 truth of this view can be established be- 



