On a new and handy Focometer. 333 



If the steady motion in question is a state of rest, u = and 

 p is a constant, so that U = 0, Y = 0, and our equation reduces 

 to the ordinary form for small spherical disturbances, 



If, on the other hand, r = co , the motion may, through any 

 finite distance, be treated as linear. We shall then have u and 

 p both constant, as well as a, and as before U = 0, V = 0. In 

 that case 



and this, by a change of independent variables, is easily seen 

 to be the appropriate form for small plaDe disturbances of a 

 fluid whose motion is otherwise uniform. 



XXXII T. On a new and handy Focometer. 

 By Professor J. D. Evekett, F.R.S.* 



THIS focometer is designed to permit the distance of the 

 " object " from the screen to be varied, while the lens 

 which is to throw on the screen an image of the " object " is 

 automatically kept midway between the two. This position, 

 as is well known, gives both the sharpest definition and the 

 simplest calculation. 



The instrument is constructed on the principle of the well- 

 known toy called lazy-tongs. A number of flat bars (fig. 1), 



Ekr. 1. 



Plan, 

 all exactly alike, are jointed together in such a way that half 

 of them are in one plane and the other half in a superposed 

 plane. With the exception of the end bars, each bar in either 

 plane is jointed to three of the bars in the other plane, one 

 joint being in the middle and one at each end. The end bars 

 are jointed at the middle and one end only. All the bars in 

 the same plane are parallel, and the two sets together form a 

 single row of rhombuses all equal and similar, a side of a 

 rhombus being half the length of a bar. The system has only 

 * Communicated by the Physical Society : read February 24, 1893. 



Phil. Mag. S. 5. Vol. 35. No. 215. April 1893. 2 A 



