100 Dr. W. Pole on some unpublished 



to be driven by rotation nearer to the longest diameter than 

 is the material in the faces. Its maximum in the positive 

 quadrant occurs at the point where 



pz=. sjab 



(71) 



+n)\ 



i.e. tan#= s/b z ja l 



where 6 is the vectorial angle. 



The corresponding maximum is given by 



a ^co 2 pa 2 a—b a i — 27)a 2 b 2 + b i ,_~v 



K7~~E~ ^ a Za* + 2a 2 b 2 + 3b*' ' ' U } 



This vanishes of course in a circular disk for all values of t) 

 and for ?? = in all forms of disks. 



§ 34. In a circular disk every line originally parallel to 

 the axis of rotation has a plane containing that axis for 

 osculating plane. Calling the radius of curvature in a line 

 at distance r from the axis of rotation R r , we easily find 

 from (67) 



a/R r =(a,> 2 /E)x(r»/«)x<l 



a/R a =(o)> 2 /E)x^(l + 7 7 ) 



Comparing (73) with (38) and (37), we find between the 

 curvature in a generator of the rim and that at the centre of 

 a face of a circular disk the simple relation 



ljn a =a/U (74) 



Thus in a thin disk the curvature produced by rotation in a 

 rim generator is much greater than that produced at the 

 centre of the faces. The curvature in the rim generator 

 might perhaps be measured by an optical method, at least in 

 a circular disk. 



X. Some unpublished Data on Colour-Blindness. 

 By Dr. William Pole, F.R.S.* 



[Plate II.] 



THESE data have reference to a paper " On Colour- 

 Blindness " which the Royal Society did me the honour 

 to publish in the Philosophical Transactions of 1859, and a 

 Report on which, by Sir John Herschel, was printed in the 

 ' Proceedings,' vol. x. p. 72. There were some points in this 



* Communicated by the Author. 



