56 Scientific Intelligence. 



too small. A plate of resin was employed of nearly the same size 

 as the glass. Small squares of thin plate glass were placed in 

 optical contact with its two faces and parallel to each other. It 

 gave evidence of irregular strain near the terminals, separated 

 the red and blue rays by a small angle, and was imperfectly trans- 

 parent. But its chief defect was that it allowed a spark discharge 

 over its surface a length of 7 inches before the distance of the 

 spark terminals much exceeds 2-5 inches. With all these deficien- 

 cies, however, it gave a regular and definite efifect, and the action 

 was contrary to that of glass. 



A plate of quartz cut perpendicular to the axis, 3 mms. thick and 

 20 long, was employed with a result similar to that of glass. — 

 Phil. Mag., 1, 337. i:. c. p. 



9. Waves on Mercury. — M. C. Dbchakme states that by blow- 

 ing through a tube touching the surface of mercury, we may pro- 

 duce a sound and circular waves forming a symmetrical network 

 upon the liquid. The smaller the interior diameter of the tube, 

 the higher and weaker is the sound and the shorter the waves. 

 As long as the diameter is less than about half a millimeter, the 

 sound resembles the buzzing of an insect. When the diameter 

 amounts to -7 or -8 mms. the sound assumes a clear and distinct 

 musical character. With larger tubes the note becomes loud. 

 There is a great tendency of the sounds to pass to their harmonics, 

 with slight changes of the pressure of the air or of the length of 

 the immersed portion of the tube, especially when the latter has a 

 diameter of 2 to 5 mms. For this reason it is difficult to obtain 

 single sounds, but thev are almost alwavs hijjh and unstable har- 

 monies. The best rf^sults are attained'with tubes 0-S to 5 mms. 

 in diameter, held vertically so as just to touch the surface, and 

 cut off perpendicular to their axes. The tube is then connected 

 with a large rubber vessel full of air and compressed by a weight 

 which should be greater the smaller the tube. 



In general the sounds thus produced depend, as regards their 

 height, quality and intensity, on the diameter, length and nature 

 of the tube, the thickness of its edges, the form of the edges of 

 the orifice, and on the temperature, pressure and nature of the 

 gas ; finally and above all, on the capacity of the reservoir of air, 

 or rather on the harmonic ratio of its volume to that of the sound- 

 ing tube and of the connecting tube. This last condition is so 

 essential that a tube giving good results with one reservoir will 

 not work satisfactorily with another. An imjiortant application 

 of this device is to the production and projection of interference 

 waves in an elliptical vessel. Tlie nodes and loops are in this 

 case clearly marked, fixed and symmetrical. The concord of the 

 third, fiflh and octave may be similarly jirojected on a screen 

 with great clearness by employing two sounding tubes. --/'>«'/•"• 



10. 'SpectUim of the light of the Urn Grotto of Capri.— Vr. H. 

 W. VoGEL, on a recent visit to Capri, tested the light of the bh"' 

 grotto with a spectroscope. As the entrance is only about four 



