Revolver Immersion Prism. By James Edmunds. 33 



structed for me by Messrs, Powell and Lealand,* which will, I think, 

 be found to render immersion illumination more manageable and 

 more generally useful. I have termed it the revolver prism, 

 because, by its means, unrefracted light at four grades of obliquity 

 may be successively thrown into the object simply by rotating the 

 prism and altering the inclination of the Microscope. This prism 

 is of hard white crown glass, and six or seven eighths of an inch in 

 diameter. Above, it has a circular plane surface, with a border 

 curving downwards so as to afford hold for a setting which does not 

 rise high enough to touch the slide. Below, it has four facets 

 produced by grinding down a spherical surface into two right- 

 angled prisms, whose lower edges are located at right angles to 

 each other, and whose faces respectively make with the top surface 

 angles of 30° and 60°, 41° and 49°. These four facets, taken con- 

 secutively, are normal to light entering at 30°, 41°, 60° and 49° 

 of obliquity to the optic axis. The prism is sprung into the top of 

 a vertical tube deeply slotted for the passage of light to the various 

 facets, each slot being cut down to a line at which the side of the 

 tube would be intersected by the plane of the facet on the opposite 

 side. Below, the tube screws or slides into an adapter, or expands 

 into a ring for the sub-stage. The top surface of the prism connects 

 to the slide by means of a minim of cedar oil or Price's glycerine, 

 and glare is prevented by the fact that superfluous light is reflected 

 out through the slot behind. Each slot is figured with the obli- 

 quity of the light for which it is cut, and by a simple addition the 

 entering light may be demonstrably limited to a particular angle, 

 as with Dr. Woodward's perforated screens. 



By means of this immersion prism the obliquity of the illumi- 

 nation may be so graduated as to shut out the light field and the 

 ordinary negative image in so far as is necessary to obtain the diffrac- 

 tion image at its best jDoint. With light at 60° from the optic axis 

 the diffraction image is so far isolated that Ampliipleura pellucida 

 in balsam may be seen upon a dark background with the new oil 

 lens. With hght at 49° or 41° the field becomes lighted in propor- 

 tion to the angular aperture of the objective, and the diatom is 

 finely displayed, but with light at 30° the lines disappear. 



Amjjliiiyleura peUuclda in air, whether upon cover or slide, may 

 also be shown by this jtrisin. If the diatom be upon the slide, an 

 intense black-ground illumination may be produced through the 

 higher-angled facets, and the lines are shown as green and black 

 bands, as they are by means of the immersion paraboloid.f If the 

 diatom be upon the cover, the two lower-angled facets will show it, 



* I exliibited this prism on June 5, 1878, at the soiree of the Metropolitan 

 Branch of the British Medical Association. 



t " On the Paraboloid Illuminator.' Vide ' Monthly Microscui>ic;il Journal,' 

 August, 1877, p. SI. 



VOL, II. I> 



