82 Transactions of the Societij. 



VI. — Note on a Bevoher Imme7'sion Prism for Svh-stage 



Illumination. 



By James Edmunds, M.D., M.E.C.R Lond., F.E.M.S., &c. 



(^Read 8th January, 1879.) 



The value of a right-angled immersion prism as a sub-stage appli- 

 ance for the illumination of objects under the Microscope was shown 

 by Mr. Wenham in the year 1855', in a paper* published in the 

 ' Transactions of the Eoyal ]\Iicroscopical Society.' Mr, Wenham's 

 paper is illustrated with a woodcut showing a right-angled prism 

 attached to the under surface of a slide by means of oil of cloves, 

 balsam, turpentine, or camphine ; light concentrated by a bull's- 

 eye being deflected upwards by means of an Amici prism. In the 

 same paper Mr. Wenham also shows how, by means of a hemi- 

 spherical lens, or " a small paraboloid of glass with a flat top " 

 similarly attached to the under surface of the slide, other methods 

 of immersion illumination may be made effective and, as he says, 

 " show the DiatomacefB with a degree of beauty and delicacy that he 

 had never seen equalled." 



The Tolles Microscopes have now for some years had fitted to 

 their stages deep spherical and cylindrical lenses to be used for 

 immersion illumination, and the splendid oil lenses now made by 

 Zeiss are sent out accompanied by a small lens to be attached to 

 the under surface of the slide with cedar oil, in order to supj)ly 

 light on the same principle. Colonel "Woodward also has re- 

 cently favoured this Society with two paners developing this most 

 valuable method of illumination for high-angled lenses, and he has 

 combined with the right-angled immersion prism two screens of 

 thin metal perforated in line with ihe. object, so that entering light 

 may, when necessary, be demonstrably limited to parallel rays at 

 a determinate angle. 



The oil of cloves, UFed as an intermedium by Mr. Wenham, has 

 been adopted by Colonel Woodward. Cedar oil, castor oil, or pure 

 glycerine (Price's) also answer perfectly. As to the hght, it will 

 be found that a 1^-inch achromatic objective serves much better 

 as a condenser than a bull's-eye, and that an image of the edge of 

 a parafiin-lamp flame should be accurately condensed upon the 

 object. 



I now have the honour to submit a new combination prism, con- 



* " On a Method of Illuminating Objects under the Highest Powers of the 

 Microscope." By F. H. Wenham, Esq. Read March 25, 1856. ' Transactions of 

 the Royal Microscopical Society,' vol. iv. pp. 55-GO. 



