THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



Vol. I. 



New York, June, 1880. 



No. 6. 



On an Improved Immersion- 

 Paraboloid. 



BY F. H. WENtA.M. 



\" 



The immersion-para )oloid was 

 :irst described by myself in the 

 } ear 1856. It was desigi ed for the 

 purpose of illuminating ( paque ob- 

 jects under the highest p( wers in, a 

 black field, by the application of the 

 principle of total reflection from 

 the upper surface of the tl in glass 

 covering the object. This surface, 

 under these conditions, served 

 as a speculum for condensin,^ 

 light on the object beneath, 

 contained in fluid, oi 

 mounted in Canada balsam. 

 In order to obtain this effect, 

 it was necessary to connect 

 the flat top o ' the truncated 

 glass paraboloid to the under 

 surface of the )bject-8lide, by 

 means of water, or preferably 

 by some more 

 highly refrac- 

 tive medium, 

 such as oil of 

 cloves, as this ' 

 allows extra 

 rays to pass 

 beyond the 

 critical angle of total reflection 

 from glass to water. This angle be- 

 tween the glass and water 's limited 

 to a convergence of abo it 120°. 

 The critical angle from glass to air 

 is near 41°, and to prcAcnt the 

 possibility of rays passing through 

 the cover at less than this angle, 

 and producing an imperfectly 



black field, I cut off the base of my 

 paraboloid a few degrees above this 

 angle, and, therefore, the figure of 

 the curve encloses an angle of 90°, 

 starting from the focal point. 



The apex, or flat top, is cut down 

 at a distance from the focal point 

 equal to the thickness of ordinary- 

 glass slides, the average of whicn 

 may be taken as about .06 inch. 

 With the paraboloid in water-con- 

 tact, the difference between 90° 

 and 120°, is an annulus of light 

 including only 15°, which is 

 all that the water will allow to 

 pass. As the risk of damage 

 to objects and apparatus from 

 using essential oils is an ob- 

 jection, I have modified the 

 immersion-paraboloid, so as 

 to obtain the fullest angle 

 admissible by using an inter- 

 medium of water only. 

 In 1856 I decided from 

 a trial of 

 other foci, 

 that an im- 

 mersion-pa- 

 raboloid of 

 ^-inch fo- 

 ""** cus gave the 

 "^ best effect, 



and as recent experiments have 

 confirmed this, I still adhere to 

 that size for general use. A A 

 Fig. 19, (drawn to the natural 

 size), is the diameter of the flat top 

 of the paraboloid. B, the focal 

 distance determined by the thick- 

 ness of the slide. This necessary 

 truncation of the paraboloid limits 



