238 



PROCEEDINGS OF THE AMERICAN ACADEMY 



quite narrow, as greater accuracy can be attained when the colors are 

 rather faint. Four points in the spectrum were selected for obser- 

 vation, and from these the intervening portions were interpolated. 

 These points were equidistant, and were situated one in the red, one 

 in the yellow, one in the green, and one in the violet ; or to speak 

 more accurately, in the neighborhood of the lines C, D, and b', and 

 at a point between F and G. They will be designated hereafter by 

 the letters R. Y, G, and V. 



I give below my observations on the lime light in full, as a fair 

 example of the accuracy of the instrument, and of the method em- 

 ployed. It will be seen that the first two figures only are of value, 

 the third being used merely for obtaining the mean result. In all my 

 experiments I divide my observations into two sets, made at different 

 times, and the light extinguished between whiles; each set is divided 

 into four series, one for each color, and each series consists of at least 

 three, and frequently more observations; thus making at least twenty- 

 four observations on each light. The means of the series are then 

 taken and compared two and two, and their means obtained. From 

 these last the relative brilliancies as compared with the standard are 

 calculated, and plotted as a curve. (See Fig. 1.) 



