the Intensities of Lights of different Colours, 



171 



Pig* 1 6 



eye was thus shielded from extraneous light and the com- 

 parisons of illuminations was made, as in experiments 1 and 2, 

 on the portions of the rings which 

 were nearest the mirror, as shown in 

 fig. 16. Vision through this screened 

 tube gave the best results. 



After practice in such comparisons, 

 made during several hours on differ- 

 ent days, I became more and more 

 skilful and the results of measure- 

 ments become more concordant ; but 

 such methods of photometry do not 

 approach the accuracy of those in 

 which two contiguous surfaces of differ- 

 ent degrees of translucency coalesce 

 into one surface of a uniform illumination, as happens on the 

 balance of illuminations on the two sides of a Bunsen-photo- 

 meter disk when these are illuminated by lights of the same 

 intensity and of exactly the same hue. A photometer for 

 this mode of observation is described in the following section. 

 It has, however, the advantage over the Bunsen photometer 

 in that it serves to measure the intensities of differently 

 coloured lights. 



The Rotating -Disk Photometer. — The photometer-disk was 

 taken apart and a ring of thin white linen paper* of the 

 diameter of the disk and -f$ centim. wide was laid on one of 

 the disks ; this was covered 

 by the circle of thin trans- 

 lucent white paper, and on 

 this was laid another ring of 

 the thin linen paper. The 

 disks were now clamped to- 

 gether. The outer portions 

 of the open sectors of the disk 

 w T ere thus closed by two thick- 

 nesses of the thin linen paper 

 with the " alba tracing-paper " 

 between them, a, in fig. 17 ; 

 while the inner portions of the 

 sectors were closed by the 

 tracing-paper alone, b, in fig. 

 17. 



On rotating the disk, it was not possible to balance the 



Rg.I7; 



* The best paper I have experimented with for this purpose is water- 

 marked " Crane & Co., Dalton, Mass., Bond, No. 21." 



N2 



