606 Captain W. de W. Ahney [May 6, 



parts of the spectrum, and the results are shown in Fig. 3 by the con- 

 tinuous curved lines, 'i he diagram would have boen too large had 

 the same scale been adopted throughout for the ordinates, each curve 

 is therefore made on a scale ten times that of its neighbour, counting 

 from the centre. 



In the diagram the sodium light of the spectrum before extinction 

 was made of lib e luminosity of the amyl-acetate lamp (hereafter 

 called A L), which is about -8 of a standard candle, at 1 foot distance 

 from the source. Before it ceased to cause an impression on the eye, 



the illumination had to be reduced to ia-qqTwTqq ^ ^^ 



65 



"Pi licrht to of its spectrum luminosity. 



jiiii^ni lo 10,000,000 ^ "^ 



150 15 



F light „ "107000,000 ^^ 1,000,000 



nr w 3000 3 



10,000,000 10,000 



^y ,, 11,000 11 



10,000,000 10,000 

 ^ ,. , 70,000 7 



Blight „ Tr.nf.ar^KaOY 



10,000,000 1000 



There was one objection which might have been oflfered to this 

 method, and that was to the use of the rotating sectors, and perhaps 

 to the ground glass. This objection was met by first of all reducing 

 the light by means of a double reflection of the beam forming the patch 

 from one or two plain glass mirrors, and also by using a plain glass 

 mirror in the box instead of a silvered glass. By this plan the light 

 falling on the first plain glass mirror was reduced, before it reached 

 the end of the box, 1000 times ; and again, by narrowing the slit of 

 the colliniator, and also the slit placed in the spectrum, another 

 similar reduction would be effected. All rays thus enfeebled were 

 within the range of extinction. It was found that neither ground 

 glass nor rotating sectors had any prejudicial efiect, and therefore 

 this extinction curve may be taken as correct. 



In the curves there are two branches at the violet side, and this 

 requires explanation. One shows the extinction when viewed by the 

 most sensitive part of the eye, wherever that may be, and the otlier 

 when the central portion of the eye was employed. The explanation 

 of this difference in perception is chiefly as follows : — 



In the eye we have a defect — at least we are apt to call it a defect, 

 though no doubt Providence has made it for a purpose — in that 

 there is a yellow spot which occupies some 6" to 8^ of the very centre 



