August 2, 1901.] 



SCIENCE. 



185 



lished by C. A. Davis * and by Blatcbley and 

 Ashley, f 



My observations upon the growth of stream 

 deposits of travertine in Tennessee and Cali- 

 fornia led me to the following conclusions : 



1. The deposits grow more rapidly in the 

 summer and at low-water stages. 



2. The channels become locally choked up 

 with travertine and the streams are compelled 

 to shift from side to side. 



3. The thin horizontal beds of travertine are 

 formed in the shallow waters immediately above 

 the falls. 



4. Although the larger streams appear to be 

 eroding and do erode at high stages and at cer- 

 tain parts of their courses, the process on the 

 whole is constructive. ' 



5. The travertine tends to form a series of 

 terraces along the streams depositing it. 



J. C. Branner. 

 Stanford University, July 4, 1901. 



PSEUDOSCOPIC VISION WITHOUT A PSEUDO- 

 SCOPE, j; 



A METHOD of securing an illusion of binocular 

 vision wholly without instrumental aid oc- 

 curred to me recently, which is interesting in 

 connection with the study of pseudoscopic 

 vision. It is fully as startling as any of the re- 

 sults obtained with the lenticular pseudoscope, 

 which I described in Science (about Nov. , 1899), 

 and, requiring the aid of no optical instrument, 

 is very much more impressive. A lead pencil 

 is held point up an inch or two in front of a 

 wire window screen, with a sky background. 

 If the eyes are converged upon the pencil point, 

 the wire gauze becomes somewhat blurred and 

 of course doubled. Inasmuch, however, as the 

 gauze has a regularly recurring pattern the two 

 images can be united^ and a little effort enables 

 one to accommodate for distinct vision of the 

 united images of the mesh. To accommodate 

 for a greater distance than the point upon which 



*Jour. of Geology, Sept.-Oct., 1900, VIII., 485-503. 



t ' The Lakes of Northern Indiana and their Asso- 

 ciated Marl Deposits,' by Blatcbley and Ashley, 25th 

 Ann. Eep. State Geologist of Ind., pp. 43-51. 



{Since writing this note I have learned that a 

 similar illusion is described in Le Conte's 'Sight.' 

 It may however be new to some. 



the eyes are converged requires practice, but the 

 trick is very much easier in this case than in 

 the case of viewing stereoscopic pictures with- 

 out a stereoscope. As soon as accommodation is 

 secured the mesh becomes perfectly sharp, and 

 appears to lie nearly in the plane of the pencil 

 point, which still appears single and fairly sharp. 

 If now the pencil is moved away from the ej^es, 

 which are to be kept fixed on the screen, the 

 point passes through the mesh and appears double, 

 the distance between the two images increasing 

 until the point touches the screen. 



If now the pencil be removed it will be found 

 that the sharp image of the combined images 

 of the gauze persist, even though the eyes be 

 moved nearer to or farther away from the 

 screen. Move the eyes up to within six or 

 eight inches of the plane in which the screen 

 appears to lie and try to touch it with the 

 finger. It is not there. The finger falls upon 

 empty space, the screen being, in reality, a 

 couple of inches further off. This is by all 

 means the most startling illusion that I have 

 ever seen, for we apparently see something 

 occupying a perfectly definite position in space 

 before our eyes, and yet if we attempt to put 

 our finger on it, we find that there is nothing 

 there. 



It is best to begin by holding the pencil an 

 inch or less in front of the screen. As the eyes 

 become accustomed to the unusual accommo- 

 dation the distance can be increased. The 

 greater the distance, the greater the illusion, of 

 course. I have succeeded in bringing up the 

 apparent plane of the mesh five or six inches, 

 but this requires as great a control over the 

 eyes as is necessary in viewing stereoscopic 

 views without an instrument. 



R. W. Wood. 



THE BOTANIST'S JOURNEY TO THE DENVER 

 3IEETING OF THE A. A. A. S. 



For the observing botanist (and what kind of 

 a botanist is he who is not observing?) the 

 journey to the Denver meeting of the American 

 Association for the Advancement of Science 

 will be of the greatest interest. Leaving the 

 originallj' wooded country some distance east 



