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V. — Immersion Illuminators. By J. Mayall, jun., F.E.M.S. 

 (^Read 8th January, 1879.) 



The need of special apparatus for illiiminatiug objects mounted in 

 balsam, or other refractive medium, seems to have been clearly in 

 Mr. Wenham's mind when he contributed his paper on " Illuminating 

 Opaque Objects " to the ' Transactions' of the Society in 1856. The 

 appliances then described were, a right-angled prism, a truncated 

 hemispherical lens, used with his paraboloid, and the " paraboloid 

 of solid glass with a flat top." These were, strictly speaking, im- 

 mersion illuminators: the last is the original "immersion para- 

 boloid." It was shown by diagrams that the illuminating rays 

 were made to impinge on the upper internal surface of the cover-glass 

 at an inclination beyond the " critical angle " (or flat-plate limit 

 between glass and air), and reflected by total rejlexion upon the 

 object, which is then seen in a dark field. 



The reflex illuminator designed by the same inventor, sixteen 

 years later, was based on the same princij)le. 



With these appliances, used according to the principle of con- 

 struction, dark-ground illumination is produced with diy objec- 

 tives, whether the illuminating rays are internally reflected from 

 the cover-glass on to the balsamed object, or the object is capable 

 of deflecting the direct rays from the illuminator so as to become 

 self-luminous and visible by means of what may be termed scattered 

 rays. 



It has been generally held that, as stated by Mr. Charles Brooke, 

 " the more minute structure of some objects is cognizable onhj by 

 its influence on rays traversing the object at considerable obliquity." 

 To this end many appliances have been designed to be used with 

 dry objectives. In Amici's prism, Nachet's prism, the truncated 

 paraboloids, right - angled prism, truncated hemispherical lens, 

 Reade's dark-ground illumination, the '' kettle-drum " diatom-prism, 

 the reflex illuminator, and others too numerous to mention, we have 

 either the use of an actual stop to block out portions of the rays, or 

 the illuminator is placed in such a position as to provide light in 

 particular directions. The main purpose in all is to utilize the 

 more obliquely incident light to the exclusion of the central. 



On the importance of regulating the obliquity of the illumination 

 on the object in its relation to the apertures of dry objectives, I 

 quote from Mr. Wenham's paper " On the Illumination of Ob- 

 jects ..."*:— 



" Practically it is found that there is a precise but different 

 angle of illumination required for every aperture of the object-glass, 

 in order to give the maximum of distinctness ; or that will even at 



♦ 'Quart. Journ.,' 1854, vol. ii. p. 152. 



