AMICI PRISM.— HEMISPHERICAL CONDENSER. 113 



and convenient one ; this consists in attaching the frame of the 

 prism to a sliding bar, which works in dovetail grooves on the top 

 of a cap that may be set on the ' secondary body beneath the 

 stage ; the slide serves to regulate the distance of the prism from 

 the axis of the microscope, 

 and consequently the obli- 

 quity of the illumination ; 

 whilst its distance beneath 

 the stage is adjusted by the 

 rack-movement of the cylin- 

 drical fitting. In this man- 

 ner, an illuminating pencil 

 of almost any degree of 

 obliquity that is permitted 

 by the construction of the 

 Stage may be readily ob- 

 tained ; but there is no 

 provision for the correction Amici's Prism for Oblique Illumination, 

 of its aberrations. In 



order to use this oblique illumination to the greatest advantage, 

 either the Prism or the Object should be made to rotate, thus 

 causing the oblique rays to fall upon the latter from every 

 azimuth in succession, so as to bring out all its markings (§ 119). 

 83. For those who desire to obtain a very oblique illuminating 

 pencil, for the resolution of the most difficult lined Tests by means 

 of Objectives of large angular aperture, without having recourse 

 to more expensive arrangements, the Double Hemispherical Con- 

 denser of Mr. Reade affords a very simple and convenient means. 

 This consists of a hemispherical lens of li or If inch diameter, 

 with its flat side next the object, surmounted by a smaller lens of 

 the same form, the flat side of which is covered with a Diaphragm 

 of thin brass or tin-foil, having an aperture or apertures close to 

 its margin. The single hemisphere originally used by Mr. Reade 

 gave an angle of convergence of about 90° for its most oblique 

 rays ; which is about the same with that of the Webster Condenser 

 as at present constructed. By the addition of the second hemi- 

 sphere, however, the angle of convergence is augmented to 150° ; 

 and its power in ' bringing out ' the lined tests is greatly aug- 

 mented. Such an arrangement, of course, involves a large amount 

 of Chromatic dispersion ; but this is stated by Mr. Reade not to 

 be a disadvantage in practice ; since with high powers the red, the 

 yellow, or the blue rays may be separately employed by altering 

 the focus of the condenser, so that the illumination becomes vir- 

 tually monochromatic. If the fineness of the lines under examina- 

 tion requires that the Condenser should be closely approximated 

 to the object, the Diaphragms may be placed between the two 

 hemispheres ; a slit in the tube being provided for that purpose. 



I 



