152 



SCIENCE 



[N. S. Vol. XXXI. No. 787 



A special device, involving an air blast, was 

 used to force the poison into the parts of the 

 plant most frequented by the adult weevils. 

 In the experiments described the application 

 was made in person by the junior author, Mr. 

 Smith, or under his personal supervision. It 

 is possible, and in fact is forcefully pointed 

 out in the report, that such successful results 

 as those obtained in some of the experimental 

 work should not be expected under the prac- 

 tical conditions on plantations. The writers 

 even point out that it is likely that nine out 

 of ten planters will fail to obtain satisfactory 

 results from the first work they do. Neverthe- 

 less, every consideration seems to indicate 

 clearly that powdered arsenate of lead can be 

 used very profitably as an important adjunct 

 in connection with the system of control that 

 has been in use heretofore. 



It is not extreme to state that the work 

 accomplished with powdered arsenate of lead 

 by Messrs. Newell and Smith marks an impor- 

 tant advance in our knowledge of the control 

 of the boll weevil. It promises in a short time 

 more than to compensate the state of Louisi- 

 ana for all the money that has been expended 

 in the operations of the Crop Pest Commission 

 since its establishment. W. D. Hunter 



U. S. Depaktmekt of Agricultuee 



SPECIAL ARTICLES 



DOUBLE IMAGES OF AN OBJECT AS SEEN THROUGH 

 A WATER SURFACE 



In Science of November 29, 1901, the pres- 

 ent writer discussed this subject as presented 

 by Matthiessen.' It was there pointed out 

 that Matthiessen's equations had all been de- 

 duced in a paper by the present writer, in 1881, 

 in the Transactions of the Academy of Science 

 of St. Louis. 



Matthiessen urged that two images of an 

 object are formed when it is viewed through 

 a water surface. One lies upon the caustic 

 of refraction, and is therefore above the level 

 of the object, and nearer to the eye. The 

 other is along the same line of sight, but on 

 the normal through the object. 



'^Ann. der Physik, 1901, No. 10, S. 347. 



In my paper of 1881 the- latter image was 

 discussed as the one actually seen. 



It is evident that all rays from a point on 

 an object thus viewed, will when produced 

 backwards, not only be tangent to the caustic 

 but will also cut the normal. Every ray of 

 the cone of rays whose base is the pupil of the 

 eye will thus appear to pass through an area 

 on the surface generated by revolving the 

 caustic around the normal. They will also 

 intersect between two limiting points on the 

 normal. The image of the point will there- 

 fore appear as distorted into an area on the 

 caustic surface, and as a short line on the 

 normal. My idea has always been that the 

 former image was too indistinct to be visible. 



Eecently, while deducing the equation of the 

 caustic, it occurred to me that the image 

 might be seen upon the caustic surface, if the 

 head were inclined so that the eyes were in 

 the same vertical plane. The axes of the two 

 cones of rays make then with each other an 

 angle lying in the vertical plane, and the eyes 

 may be focused on their point of intersection. 

 The images on the caustic will then be prac- 

 tically superposed, and the line images on the 

 normal will be more widely displaced on each 

 other. The experimental result is very stri- 

 king, and may easily be obtained by observing 

 a chain, or the water-plug and chain at one 

 end of a bath tub filled with water. 



When both eyes are used, the water plug 

 with the vertical chain, to which it is attached, 

 appears projected towards the observer by a 

 foot or more, if the eyes are near the surface 

 and at the opposite end of the bath tub. If 

 one eye be now closed, the image recedes to 

 the vertical line through the object, appearing 

 along the same line of sight as before. It 

 therefore appears at a lower level. 



When both eyes are in the same horizontal 

 plane, the image is seen on the normal through 

 the object. The images on the caustic surface 

 as seen by the two eyes are then displaced on 

 each other, and those on the normal coincide. 

 Opening and closing one eye then produces 

 no change in the position of the image. 



Francis E. Niphee 



