36 Transactions of the Society. 



VII. — A Catoptric Immersion Illuminator. 

 By John Ware Stephenson, F.KA.S., Treas. E.M.S. 



[Read 8th January, 1879.) 



As the subject of Immersion Illuminators is now before the Society 

 (and I am very glad it is so, for without their help the full resolving 

 powers of the recent large-angled objectives cannot be utilized), it 

 may not be out of place to lay before the Fellows a brief account of 

 an immersion condenser of very simple construction which I devised 

 in 1877. 



The diagram shows the form and size of the instrument which 

 I now use, although it is sufficiently obvious that other sizes, 

 in the same ratios, may easily be made — in 

 fact, I have such. 



The apparatus is simply a plano-convex 

 lens, worked on a 1-inch tool, and having a 

 diameter of 1 • 2 inches, which is then " edged " 

 down to 1 inch, as being more convenient in 

 size, and as giving an aperture sufficient for 

 our purpose. 



The upper, or convex side, of the lens is 

 cut down or flattened, so as to give a surface y\ of an inch in 

 diameter, with Avhich the slide is to be connected, when in use, by 

 a drop of oil or water. 



It matters not which fluid is used as long as the objective has a 

 numerical aperture not exceecling 1'33 (the index of water), and 

 it is very improbable that this will ever be exceeded to any great 

 extent, as 1 ■ 50 is the ideal maximum of even an oil immersion. 



The upper curved surface of the lens is silvered, and beneath 

 the lens, a flat silvered plate -^^ of an inch thick, and correspond- 

 ing in size and position with the upper flattened surface, is balsamed. 

 It will be seen that the incident ray is normal to the under 

 surface, impinges on the curved silvered surface, and is thus thrown 

 back on the plane, or under surface of the lens, whence the more 

 oblique rays, falling beyond the critical angle, are totally reflected, 

 and converge to a focus, giving a numerical angle of 1 '30 = 120° 

 in balsam. 



The object of placing a silvered glass disk beneath the lens is 

 twofold : in the first place, it reflects the less oblique rays which 

 fall within the critical angle, and in the second it tends to diminish 

 the spherical al;erration which in this zone might otherwise be felt. 

 The stop is placed about \ of an inch, or less, below the con- 

 denser, and the opening used is of a lens-shaped form, as giving 

 a broad beam without any appreciable spherical aberration in so 

 narrow a zone of light. 



