Measuring Aperture. 283 



posterior lens additions, as these give rise to endless confusion and 

 disagreement. Professor Smith does dispense with them altogether 

 in his apertometer. 



MEASURING APERTURE. 



A LETTER FROM MR. JOHN MAYALL, JR. 



{^Received June 6th, iSjg.) 



Prof. H. L. Smith, in his communication on " Angular Aper- 

 ture, &c.," in your issue of April, alludes to an experiment with 

 an objective, a }i inch of Spencer, made when the systems were 

 closed and a spot of ink placed on the flat surface of the front 

 lens, just large enough to cut off the little circle of light that ap- 

 pears when one looks into the objective, with the front system 

 toward the eye. Under these circumstances he was still able to 

 see light by swinging the sector arm up to 179°. I fail to see the 

 value of this experiment, and when further on I find Prof. Smith 

 says that the "true air angle at the same closed point, measured 

 by the angle of the triangle whose apex is the focal point, and 

 whose base is the diameter of the spot of ink which just stopped 

 out the light, is only 144"," I am forced to the conclusion that he 

 has not, in this instance, been seriously attempting to measure 

 aperture. 



It being admitted that no aperture, properly speaking, can be 

 measured unless the image of a point be rendered, approximately 

 at least, as a point ; does Prof. Smith mean to say that when the 

 system of lenses is closed he can measure the true air angle ? My 

 experience with immersion lenses of high angle, is that when the 

 system is closed, so as to get definition through the thickest cover- 

 glass and the immersion medium, at that adjustment no true air 

 angle can be obtained. The true air angle can only be actually 

 measured when the objective is adjusted so as to give true focus 

 in air, a focus sensibly free from aberration. 



With regard to Prof. Smith's suggestion for a universal aperto- 

 meter, in so far as the appliance is adapted to immersion lenses, 

 it appears to me that Mr. Tolles's traverse-lens is practically the 

 same thing ; bull's-eye, sector, and eye-lens in sliding tube. 



Prof. Smith finds his results corroborated for wide angles by 

 Abbe's apertometer, saying " the agreement between the two 



