Mountain-Sickness ; and Power and Endurance. 47 

 on the mixture curves in the Complementary Region is 



The equation for determining points on the mixture curves 

 in the Imaginary and Abnormal Regions has just been shown 

 to be (for one part of the curve) 



S + ngSg^iSx+afW, 



and it is obvious that for the other part it will be 



S + n 1 S 1 = ?i 2 S 2 + ^W. 



3. In defining a colour as compounded of a spectrum 

 colour and white, we have to determine both quality and 

 quantity of the white light. 



A colour may be defined by three spectrum colours by 

 means of the equation 



rifti + n 2 $ 2 = W3S3 + #W, 

 or of the equation 



n 1 $ 1 + ft 2 S 2 + n 3 S 3 =-xW. 



The colour is that obtained when a patch formed in ac- 

 cordance with, one side of the equation is made identical with 

 a patch formed in accordance with the other. In this case 

 quantity of white need not be defined, but quality must be 

 defined. 



If a colour be defined by four spectrum colours the defini- 

 tion is independent of any convention as to the quality of 

 white light. Let S l7 S 2 , S 3 , S 4 be four spectrum colours 

 whose wave-lengths are in order of magnitude, then the 

 equation 



f^Sj + W 3 S 3 = ?l 2 $2 + ^4^4 



defines a certain colour, which can be obtained by making a 

 patch of S x and S 3 identical in colour with one of S 2 and S 4 . 



IV. Mountain- Sickness ; and Power and Endurance. 

 By R H. M. Bosanquet, F.R.S* 



THE account of mountain-sickness at great altitudes, 

 given in Mr. Whymper's work, ' Travels amongst the 

 Great Andes of the Equator/ is of great interest. On reading 

 it, it appeared to me to suggest a simple explanation of the 

 occurrence of mountain-sickness under the circumstances 



* Communicated by the Author, 



