508 iidnation of Cases of Tobacco Scotonia, $c. [May 



The orange sodium light of the spectrum was thrown on the 

 apparatus therein described, of a luminosity of an amyl lamp 1 foot 

 off, and the slit giving this brightness remained unchanged through- 

 out the examination, and was moved through the spectrum till a 

 position was reached where all light was just extinguished. Her per- 

 ception of the point of extinction was very acute. Rotating sectors 

 were placed in front of the apparatus, as described in the paj 

 referred to, set at different angles, so that the amount of reduction 

 the luminosity of the spectrum was known at once. 



Scale readings of light extinction. 



The extinction of light with the full aperture to myself was at 2'1 

 and 57'9. At 57'9 the luminosity of the spectrum is 0*22 that at the 

 D line, and as the light on the screen at the end of aperture is 1/620 

 that falling on the instrument originally, it follows that the extinction 

 to myself was when the light of 57'9 (\ 6510) was 0'22/620 = 

 0'000355 of an amyl lamp placed at 1 foot from the screen. These 

 details are given to show that the newer instrument used in these 

 tests gives the same results as the older one, for with the latter it was 

 0-000350. 



The place in the spectrum where Miss W. last perceives light 

 is the same as my own. The luminosity which is invisible to her 

 is when 1'45/100,000 of an amyl lamp illuminates a screen 1 foot off. 

 At D if 71/100,000 of an amyl lamp illuminated a screen I foot off 

 it is invisible to her. With my own vision if the screen be illumi- 

 nated with 7/100,000 of the same light it just becomes invisible. 

 There is therefore a marked difference between the two sights as 

 regarding initial perception of light. 



