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XYIII. — Reply to tlie foregoing Note. 

 By F. H. Wenham, F.K.M.S. 



{Read 9th April, 1879.) 



As those who have been engaged in the aperture controversy have 

 explained their meaning repeatedly, I quite agree with ^Yhat I 

 understand is the view of the Council, that it should now be 

 closed till some new fact appears to elucidate the question. 



Professor Keith's Note does not call for discussion, as the ob- 

 jections appear to arise from a misapprehension of the acting 

 conditions of the sector measurement. The flame does not remain 

 in the centre of the field of the eye-piece during the traverse, and 

 there is no axial bisection ; the least movement sideways causes the 

 image of the lamp to leave the centre, and when at last the light 

 margin divides the field, the half illumination is actually caused 

 from the eclipse of the light by the edge of the eye-piece stop. 

 The position of the distant flame can be seen with an '• examining 

 lens " over the eye-piece. The field is traversed by the beam of 

 light ; this successively intersects all the oblique pencils of the 

 object-glass which afterwards enter together in proximity at the 

 eye-piece at a very small angle of divergence. 



The sector measurement fails to indicate true angles of 

 aperture, and in order to prove this without theorizing, I de- 

 scribed in my last paper a plain and unmistakable demonstration. 

 I took a series of decisive angles of aperture by the '' triangle " 

 method, viz. from the focal distance up to a definite diameter of 

 front lens ; I then measured the angle from each of these restricted 

 diameters or apertures by the sector, employed precisely in the 

 ordinary manner, and tabulated the comparative results as " false 

 apertures." 



With this I am content to allow all personal controversy to 

 remain at rest, as I consider that I have clearly proved that angle 

 of aperture is usually measured greatly in excess, as angle of field. 



