136 Transactions of the Society. 



computed the aperture to be about 1 1 0^. We have Professor Stokes's 

 authority for the vahdity of these computations. 



As to practical measurements : we have the testimony and report 

 of Dr. Woodward, Professor S. Newcomb, and Professor Keith on 

 behalf of a four system 3^ by Tolles. We have Dr. Woodward's re- 

 port of the measurement of the xV to which the earlier computation 

 referred; and I exhibited the measurement of the \ before the 

 Society, with Professor Abbe's apertometer. In all these cases aper- 

 tures were recorded beyond the limit contended for by Mr. Wenham. 



I felt bound to exhibit the actual measurement of the \ to which 

 the newer computation referred ; at the same time I was provided 

 with twelve other immersion lenses by Tolles, Powell and Lealand, 

 and Zeiss, all of which would have afforded similar proof. 



I do not attempt to follow Mr. Wenham in his various sug- 

 gestions for angle measuring. I have found the results obtained 

 with Professor Abbe's apertometer confirmed by a modification of 

 Professor Eobinson's plan of measuring (adapted for immersion 

 lenses), and therefore, until Mr. Wenham can show some material 

 error likely to arise from the proper use of the apertometer, I shall 

 continue to regard it as a convenient and reliable appliance. 



With regard to the supposed effect of the " outer oblique rays 

 extending to the margin of the field," Professor Keith's computa- 

 tions refer only to the central pencil — have nothing to do with any 

 appreciable diameter of field. If apertures be measured by means 

 of a small pencil of sunlight from the eye-piece, the diameter of the 

 field at the front focus is almost inappreciable, and therefore no 

 question can possibly arise concerning " outer oblique rays " : this 

 has been done in many cases to test the accuracy of the aperto- 

 meter method. 



