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lenses was mounted. On account of its precise analogy with the 

 iris of the human eye, Professor Abbe called this particular aper- 

 ture- the iris of an optical system, and then went on to show the 

 importance of its position in determining the action of optical 

 instruments. 



Little more than a few hints were to be found in the usual 

 books, but he would try to show how the position of the iris 

 affected microscopical perspective. As a diminution of the size of 



the iris would only cause a sharpening of those parts of the image 

 which corresponded to points of the object not in exact focus, 

 without altering the size of the image, it was permissible to 

 consider the centre of the iris as if the latter were really a mere 

 pinhole. If they now referred to Fig. 1 they would notice that 

 here the iris was practically in contact with the object-glass — the 

 latter being represented by a simple lens. Tracing the image of 



Fig. 2. 



two equal-sized objects, 1 and 2, they would find that in this case 

 the nearer object yielded the larger image, in accordance with the 

 ordinary rules of projection. If they now turned to Fig. 2, where 

 the iris I was placed well behind the lens — in fact, behind the 

 upper focal plane — they would find a totally different state of 

 affairs. A pinhole so placed could only pass rays which had 

 crossed in the conjugate point I'. Now, drawing such rays from I' 

 through the extremities of the equal-sized objects 1 and 2, they 

 would find the images 1 and 2 reversed in size as compared with 



