THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL 



YoT. I. 



New York, June, 1880. 



m. 6. 



\" 



On an Improved Immersion- 

 Paraboloid. 



BY F. H. WPmHAM. 



The immersion-paraboloid was 

 first described by myself in the 

 year 1856. It was designed for the 

 purpose of illuminating opaque ob- 

 jects under the highest powers in a 

 black field, by the application of the 

 principle of total reflection from 

 the upper surface of the thin glass 

 covering the object. This surface, 

 under these conditions, served 

 as a speculum for condensing 

 light on the object beneath, 

 contained in fiuid, or 

 mounted in Canada balsam. 

 In order to obtain this effect, 

 it was necessary to connect 

 the flat top of the truncated 

 glass paraboloid to the under 

 surface of the object-slide, 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 

 from glass to water. This angle be- 

 tween the glass and water is limited 

 to a convergence of about 120°. 

 The critical angle from glass to air 

 is near 41 °, and to prevent the 

 possibility of rays passing through 

 the cover at less than this angle, 

 and producing an imperfectly 



black field, I cut ofi: 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 which 

 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 

 trial of 



a 



other 



that 



• foci, 

 an im- 



of total reflection 



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 



