400 Mr. L. Schwendler on a 



measurable quantity of light. Compare the results of experi- 

 ments 1, 2, and 3, and of 6, 7, 8, and 9. 



Covering the glass covers inside with black paper to avoid 

 back-reflection, appears to weaken the light, as was to be ex- 

 pected. Compare the results of experiments 1, 4, and 5. 



i 

 The ratio - of the two lights is independent of the strength 



of the current, which it ought to be. 



These results, although showing nothing extraordinary, i. e. 

 what could not have been foretold without making the experi- 

 ments, are nevertheless valuable, since they prove, in the first 

 place, that thin glass covers take away very little light, and 

 that back-reflection is also very little ; but small as these in- 

 fluences are, they have been unerringly measured by the pho- 

 tometer, showing this instrument to be very accurate and the 

 eye quite trustworthy. That the light i, produced by P. L. S. 

 III., was so much more intense than i! , produced by P. L. S. 

 II., is due to the fact that the platinum sheet out of which 

 no. II. was cut was much thicker than the other. 



Detailed Description of the Standard and the Method of using it. 



Fig. 9, Plate XI., gives the construction of the platinum 

 standard in half its natural size. I need not give further ex- 

 planation on this point, as every thing will be readily understood 

 from the drawing. 



Fig. 10 shows the diagram of the connexions : — 



P. L. S. is the standard. 



G, a current-indicator, or, better, current-measurer. The 

 deflecting-ring must consist of a few convolutions of thick 

 copper wire, of no perceptible resistance. The small magnet 

 needle is best pivoted, carrying a long aluminium index. 



E is the battery, consisting of a few elements of high E. M. F. 

 and low internal resistance connected up successively. Grove's, 

 Bunsen's or large Daniell's cells will answer well for the 

 purpose. 



(1) is a stopper, by which the circuit can be conveniently 

 opened or closed. 



M is a mercury rheostat of about one unit resistance. A 

 groove of about 1 millim. section and 1 metre total length is 

 cut in hard wood (not ebonite, as mercury does not run well 

 in ebonite). The hard wooden board is supported by three 

 levelling-screws. 



Further, the mercury is in perfect metallic contact with two 

 iron terminals,//. These terminals are not to be fixed to the 

 board. They are simply placed in the mercury, which fills 

 small reservoirs at each end of the mercury thread. 



