122 THE ADDRESSES, LECTURES, ETC., OF 



Light, which may be divided into two principal sections, that 

 including the measurement of the wave-length of lights of diffe- 

 rent colours, and the angle of polarization, which belongs purely 

 and entirely to physical science ; and the measurement of the 

 intensity of light by photometry, which, while involving also 

 physical problems of the highest order, has an important bearing 

 also upon applied science. The principal methods that have been 

 hitherto employed in photometry are by the comparison of shadows, 

 that of Eumford and Bouguer ; by employing a screen of paper 

 with a grease-spot, the lights to be compared being so adjusted 

 that the spot does not differ in appearance from the rest of the 

 paper, Bunsen's method ; Elster's, by determining in combustion 

 the amount of carbon contained in a given, volume of a gas ; and 

 the one lately introduced by Prof. Adams and Dr. Werner 

 Siemens, by measuring the variation in the electrical resistance of 

 selenium, under varying intensities of light. 



Before concluding, I wish to call your attention to two measur- 

 ing instruments which do not fall within the range of any of the 

 divisions before indicated. The first is an apparatus designed 

 chiefly by my brother, Dr. Werner Siemens, by which a stream 

 composed of alcohol and water, mixed in any proportion, is 

 measured in such a manner that one train of counter wheels 

 records the volume of the mixed liquid ; whilst a second counter 

 gives a true record of the amount of absolute alcohol contained 

 in it. The principle upon which this measuring apparatus acts 

 may be shortly described thus : The volume of liquid is passed 

 through a revolving drum, divided into three compartments by 

 radial divisions, and not dissimilar in appearance to an ordinary 

 wet gas-meter ; the revolutions of this drum produce the record 

 of the total volume of passing liquid. The liquid, on its way to 

 the measuring drum, passes through a receiver containing a float 

 of thin metal filled with proof spirit, which float is partially sup- 

 ported by means of a carefully adjusted spring, and its position 

 determines that of a lever, the angular position of which causes 

 the alcohol counter to rotate more or less for every revolution of 

 the measuring drum. Thus, if water only passes through the 

 apparatus the lever in question stands at its lowest position, when 

 the rotative motion of the drum will not be communicated to the 

 alcohol counter, but in proportion as the lever ascends a greater 



