On an Incandescent Paper-carbon Horseshoe Lamp. 21 



active liquid, say sugar solution. This cell is interposed in 

 the path of the pencil ; and in it is inserted a piece of plate 

 glass several millims. thick, arranged so that one half the pen- 

 cil passes through it. This half therefore passes through a 

 less thickness of the active substance than the other half, and 

 is less rotated. The two then emerge as before, having their 

 planes of polarization slightly inclined to each other. This 

 inclination, and consequently the sensitiveness of the instru- 

 ment can be varied either by varying the strength of the active 

 solution, or the thickness of the plate of glass inserted in the 

 cell. 



This arrangement, as far as it lias been tested, gives as good 

 results as the previous one, while it is much more easily con- 

 structed and adjusted. 



III. Measurements of an Incandescent Paper-carbon Horse* 

 shoe Lamp constructed by Mr. T. A. Edison. By Henry 

 Morton, Ph.D., A. M. Mayer, Ph.D., and B. F. Thomas, 

 A.B* 



IT may seem almost superfluous to describe the carbon 

 horseshoe electric lamp as recently constructed by Mr. 

 T. A. Edison, so much has been written about it in journals 

 of all descriptions from the daily papers upwards ; but to 

 make our work complete we will state 

 briefly that the lamp measured by us, 

 and represented in the accompanying 

 cut, consists of a pear-shaped glass 

 globe with two reentering tubes at its 

 smaller end, through which are passed 

 platinum wires with little screw-clamps 

 at their upper ends, which hold the 

 ends of the carbon horseshoe. 



This horseshoe is 1*18 inch high, 

 and 0'72 inch across at the widest 

 part. 



It is made by charring a piece of 

 thin cardboard of similar shape, out of 

 contact with air. The interior of the 

 globe is very perfectly exhausted. 

 Fine copper wires connect the plati- 

 num wires with the binding-screws on 

 the wooden base of the lamp. 



The present writers believe that the following measurements, 



* Communicated by the Authors. 



