118 Angular Aj^eriure of Ohjed-gJasses. 



The aperture is then measured in the usual way. If the slit is very 

 narrow the limits of aperture will be known by the sudden dis- 

 appearance of light. If it is opened wider so that the edges just 

 appear in the field of view, the boundary of the circle of light limit- 

 ing the aperture may then be seen to bisect the centre ; it is there- 

 fore preferable to open the slit till the edges appear in the margin 

 of the field. 



It may not be necessary to fit up an arrangement for balsam 

 apertures, because this must always follow a true laio relative to the 

 other, and cannot exceed 82^ in ordinary object-glasses, to be also 

 used dry, notwithstanding all that has been alleged to the contrary 

 (for my case is not disproved, my friends, and I have not yet " come 

 off second best "). 



I wdll now briefly refer to the plan whereby I first obtained the 

 full dry aperture on an object immersed in balsam. Firstly, the 

 whole aperture primarily exists in the object-glass proper ; the 

 hemisphere cemented over the object had nothing to do with any 

 increase optically as a question of refraction, for it exerted no refrac- 

 tion whatever. Whether the sphere was of longer or shorter radius 

 made no difierence, provided the object lay in its centre. It acted 

 in either case for a radiating pencil, just as a piece of plane glass 

 acts for a 'parallel beam, and neither magnified or diminished. The 

 combination was not strictly an immersion object-glass, for the hemi- 

 sphere might be said rather to belong to the object, and as part of it, 

 forming together a dry element with a structure in its centre. To 

 make it an immersion, in the true sense of the term, insert water, or 

 say balsam, of the same refractive power as the hemisphere, between 

 it and the front of the object-glass proper, and then it becomes a 

 simple immersion, with its correspondingly reduced aperture. With 

 balsam the hemisphere is absolutely nil in effect, which remains the 

 same, whether it is there or not.* 



As my opponents hold trigonometrical demonstration so lightly, 

 I will recapitulate the conditions in somewhat unscientific terms. 



♦ Tlie experiments with au additional hemisplierical lens set over the objects 

 were tried by me many years ago for tlie purpose of ascertaining its practical value 

 for the exclusive investigation of balsam-mounted objects requiring large aperture 

 (as an addition for improving a properly.corrccted dry lens it is useless). The same 

 lens witli the object in tlie centre of radius would serve for any object-glass in 

 which there was sufficient distance before the front lens to introduce it. Eighths 

 and upwards of large aperture come so near that it is scarcely possible to make 

 any use of it, as an extra adaptation, .suitably mounted in another setting in 

 front. With a |th or -^th not mucli exceeding 100^, tliere is ample space. Finding 

 the effect but little superior to that produced by a higher power in the ordinary 

 way, I did not trouble the makers about adoi^ting the jjlan, which I should have 

 done bad it proved of sufficient utility. I was then aware of the optical conditions 

 involved, and published them at the time, and did not abandon it without due 

 experiment. I am stating the candid fact, not with the view of disparaging any 

 present attempt, though this in all probability will now be the motive attributed 

 10 me. 



