18 Prof. J. H. Poynting on a simple 



partment stands for what is neither X, Y, Z, nor W, or 

 X Y Z W. The second figure represents a horizontal section 

 through the middle of the instrument. Each of the ellipses 

 here is, in fact, a section of an elliptical cylinder, these cylinders 

 intersecting one another so as to yield sixteen compartments. 

 Each compartment has a wooden plug half its height, which 

 can move freely up and down in the compartment. When 

 the machine is ready for use each plug stands flush with the 

 surface, being retained there by a pin ; we therefore have the 

 appearance presented in fig. I. When we wish to represent 

 the destruction of any class, all we have to do is slightly to 

 draw out the appropriate pin (the pins of course are duly 

 labelled, and will be found to be conveniently grouped), on 

 which the plug in question drops to the bottom. This, of 

 course, is equivalent to the shading of a subdivision in the 

 plane diagram. As the plugs have to drop independently of 

 one another, a certain number of them, it will be seen, have 

 to have a slot cut in them, so as to play free from the pins be- 

 longing to other plugs. When the plugs have to be returned 

 to their places at the top, all we have to do is to turn the in- 

 strument upside down, when they instantly fall back, and on 

 pressing in the pins again they are retained in their place. 

 The guards outside the pins are merely to prevent them from 

 being drawn entirely out. 



II. On a simple Form of Saccharimeter. By J. H. Poyn- 

 ting, Fellow of Trinity College, Cambridge, Professor of 

 Physics in Mason'' s College, Birmingham*. 



THE general principle of the modification of the sacchari- 

 meter which I shall describe in this paper is well known, 

 and has already been applied in the construction of several 

 standard instruments, such as Jellett's and Laurent's. This 

 principle consists in altering the pencil of rays proceeding 

 from the polarizer in such a way that, instead of the whole 

 pencil having the same plane of polarization, the planes of 

 the two halves are slightly inclined to each other. The ana- 

 lyzer is therefore not able to darken the whole field of view at 

 once. In one position of the analyzer the one half of the field 

 is quite dark ; in another position, slightly different, the other 

 half is dark ; while when the analyzer is halfway between 

 these two positions, the two halves of the field are equally illu- 

 minated. This will be seen from the accompanying figure. 



* Communicated by the Physical Society. 



