lUO CORRESPONDENCE. 



collect image-fonuing rays," the onus prohundi rests on him. It will 

 not do to tell us he has repeatedly given simple demonstrations of 

 the truth of this jiroposition. I have shown that he has been lax in 

 testing his own demonstrations, — that with the very lens reported 

 upon, an immersion ajoerture ujjwards of 8° beyond the limit he 

 contends for has been measured by himself — with his own method — 

 in my presence. His former measurement must therefore be rejected, 

 and Dr. Woodward, Professor Keith, and Mr. Tolles may claim this 

 practical demonstration in proof of their position. 



I would request those who are interested in the subject to observe 

 that in Mr. Wenham's published account of his measurements of the 

 aperture of the gth lens by Tolles — to which I have above referred — 

 several pages are devoted to an irrelevant and misleading report con- 

 cerning the aperture of the lens when used dry. It being admitted 

 that no image-forming aj^erture can be measured unless the lens be so 

 adjusted as to give a true image of the glass surface of the cube or 

 semi-cylinder, it is absolutely essential that means be taken to verify 

 whether or not this has been done. What then is the value of his 

 report on the air-aperture of this lens when it becomes known that at 

 no point of adjustment will it give a true image of an uncovered 

 object ? The lens was designed for immersion ; it will not focus on 

 an uncovered object ; when, therefore, he set the lens at " closed " — or 

 maximum angle — ^by what process did he determine the focal distance 

 to be "013 inch, so as to enable him to state the air-aperture to be 

 necessarily less than 118°? 



With such a front lens as that of Mr. Tolles, the aberration if used 

 on an uncovered object is so enormous that there is really no such 

 thing as an air-aperture ; and by taking this or that point as the best 

 focal -distance, and this or that point as the best adjustment of the 

 lens, Mr. Weuham might have made the air-aperture anything within 

 180°. Apart from the question of aberration, he might have measured 

 the air-aperture to be any angle from about 110° to— say — 175°. 



It remains that I express my willingness to let the trial of the im- 

 mersion aperture of this ^th lens be repeated at the Society's rooms 

 — strictly according to the terms stated in Mr. Wenham's report — on any 

 evening that may be appointed by the President of the Society. 



I am. Sir, your obedient servant, 



John Mayall, jun. 



Courtesy in Correspondence. 



To the Editor of the ' MoidMy Microscopical Journal.' 



Hartley Court, Eeading, January 14, 1876. 

 Dear Sir, — I doubt whether the objects of the Eoyal charter 

 granted to the Society (to which many esteem it an honour to belong) 

 can be said to be furthered by the style of letters sometimes commu- 

 nicated to the Journal. 



As for those letters recently published by Messrs. Hogg, Brooke, 

 and Wenham, J hope and would fain believe that these* gentlemen 



