270 Transactions of the Socieiy. 



XVII. —Note on Mr. Wenham's paper " On the Measurement of the 

 Angle of Aperture of Objectives." By Professor K, Keith. 



(Read 12th Fehruary, 1879.) 



Mr. Wenham, in a paper read November, 1878,* seems to appre- 

 hend the interference of outside light in the ordinary process of 

 measuring angular aperture, but his attempt to explain this 

 interference fails to show where his difficulty is. His figure has no 

 meaning in connection with the subject, as the outside legs of the 

 tripod will not after refraction come to the same point in the field 

 of the Microscope that the central one does, and therefore have 

 nothing to do with the measurement. They will, of course, after 

 refraction fall far to the right and left of the centre, and have 

 nothing to do with the aperture question. One point in the image 

 corresponding to one point in the object, and one only, is to be 

 considered in making the measurement for aperture. It is true 

 that with the sector as ordinarily used, first one edge of a lamp 

 flame is brought to the centre of the field, and then the other edge : 

 but the few minutes of arc subtended by the flame are, strictly, to 

 be subtracted from the reading of the sector, thus practically 

 making one edge of the flame only the object of consideration. 



Those interested will bear in mind that in measuring aper- 

 ture with the sector, the lamp flame is placed far enough away 

 to render the rays of light sensibly parallel. The Microscope tube 

 is then inclined to the direction of the lamp flame, until the outside 

 ray of the flame is bent along the axis to the centre of the field, and 

 the sector read. The tube is then again inclined to the direction 

 of the flame on the opposite side until the outside ray of the flame 

 is again bent along the axis to the centre of the field, and the sector 

 again read. 



Half the difference of the readings gives practically the exact 

 amount by which the ray of light is bent. The whole difference is 

 under these circumstances the angular aperture, and if two lights 

 be placed so that their directions will form that angle at the 

 objective, both lights will be visible at the same time in the Micro- 

 scope. Whether the lights give but a single ray or a large bundle 

 of parallel rays, the result will be the same. Any allusion to 

 outside rays as interfering in this simple process is therefore 



erroneous. 



* Vol. i. p. 321. 



