208 B. DAYDON JACKSON ON AN IMMERSION PARABOLOID. 



allied to that order of the Hydroid Ccelenterata known as the 

 Thecaphora or Sertularina. 



The specimen which has formed the subject of these remarks 

 was collected by the Geological Survey of Scotland, at Leadhills, 

 Lanarkshire, along with a series of fossils which parallel the rocks 

 of this locality with those of Moffat, Dumfriesshire, or with the 

 Llandeilo Flags of Wales. 



On an Immersion Paraboloid. 



By B. Daydon Jackson. 



(Communicated May 26th, 1871.) 



I have brought for exhibition an immersion paraboloid, thinking 

 that a comparison of its powers and mode of working might be in- 

 teresting to the members of the Club. In using the ordinary 

 form of the paraboloid with the higher powers, for which it is in- 

 tended — say, for instance, the four-tenths or quarter inch glasses — 

 the moveable stop must be brought so near the object that the 

 extremely oblique rays, coming from the very top of the illuminator, 

 instead of passing through the slide by refraction, do not enter at 

 all, but are reflected down on the stop which, in spite of its black- 

 ened surface, becomes capable of sending up so much light as to 

 render the field of a neutral tint, or even of a light grey. Some 

 of these defects are avoided by using the instrument to which I 

 have called your attention. It is formed of a solid paraboloid of 

 glass, ground to a different curve than the dry form, and instead of 

 having its emergent surface hemispherically hollowed out, it is left 

 nearly flat, a very slight concavity only being given. This con- 

 cavity is so slight as to be hardly perceptible, but is intended to 

 permit the slide in contact with it, by means of the water film, to 

 be moved to and fro without danger of scratching the glass top of 

 the illuminator — no very difficult thing to do, in spite of the ap- 

 parent hardness of the substance. The stop to prevent direct rays 

 passing into the microscope is cemented to the lower surface of the 

 paraboloid. The object (Eupodiscus argus) is shown by a quarter- 

 inch binocular with a black field; the angle of the object glass 



