160 



Capt. W. de W.'Abney on 



[Mar. 7, 



observations are singularly accordant. The following observations 

 made with a standard candle and a paraffin lamp, the light traversing 

 a red medium, will give an idea of the correctness of the results : : — 



Readings of the Lamp in Candle Power. 



10-25 

 10-23 

 10-18 

 10-23 

 10-26 

 10-23 



Mean 1023 



If there be any drawback to this method, it is that an assistant is 

 required to alter the distances of the movable light. 



I may also remark that there are shades of light between which it- 

 is sometimes difficult to judge. If too bright the requisite diminution 

 can be obtained by using diaphragms in the telescope, and by altering 

 the magnifying power of the eye-piece. If too dark, an addition to 

 the light which reaches the slits may be given by giving them an 

 equal amount of illumination from an independent source. 



To secure the separation of the different portions of the spectrum, 

 resort was had to absorbing media. After many trials I came to the 

 conclusion that nothing seemed preferable to red glass, which absorbed 

 practically every part of the spectrum except the red end, and an 

 ammoniacal solution of copper sulphate. These two absorbing media 

 were employed in the experiments which I am about to describe ; very 

 little of spectrum is left unaccounted for, and the results obtained are 

 sufficiently striking to record. 



A red glass was obtained which had sufficiently plane surfaces, and 

 a cell with parallel sides was used for holding the copper solution. 

 The absorbing media were placed between the rod and the opaque 

 screen, and absolutely parallel with the latter. 



The following is an outline of the experiments to which the atten- 

 tion is to be drawn. The primary object was to ascertain the relation 

 between the horse-power expended, the light produced, and the number 

 of revolutions of the armature. The machine employed was of the 

 Gramme form, and weighed somewhere about 17 cwt. It had three 

 pairs of vertical coils, one of which was used for magnetization. To 

 work it, what is known as the steam sapper was employed. This is a 

 road locomotive from the shops of Aveling and Porter, of Rochester, 

 and is adapted to driving machinery by applying a band to the fly- 

 wheel. It works fairly steadily when not overtaxed. An indicator 

 was attached for taking diagrams from which to calculate the horse- 

 power expended. The number of revolutions of the armature was 



