THE 



MONTHLY MICROSCOPICAL JOURNAL. 



JUNE 1, 1872. 



I. — On an Improved Bejiex Illuminator for the Highest Pouers 

 of the Microscope. By F. H. Wexham. 



(^Read before the Eoyal Microscopical Society, Mai/ 1, 1872.) 



Some retrospect may be needed to explain my reappearance before 

 yon on a subject whicb by tbis time may perbaps be considered 

 threadbare and exhausted, but the very singular effects of the 

 improved arrangement will, I tnist, exonerate me from all undue 

 intrusion. 



It is now sixteen years since I described in a paper read before 

 this Society (^larch 26, 1856) a mode of illuminating objects under 

 the highest powers of the microscope. As then explained, this was 

 strictly an opaque illumination, intended only for such objects as 

 were mounted in balsam or fluid, as the intermedium permitted the 

 shde and cover to be treated as an entu-e refractive plate, the upper 

 plane of which, or cover, could thus be used like a speculum ])y 

 throwing light down again from it as a tutal reflecting surface on 

 to the objects beneath. 



Several methods were figured and shown at the meeting for 

 allowing rays above the angle of 41^ to pass direct to the cover, 

 through which of course they could not escape, but by the law of 

 total reflexion would be thrown back again and intercepted by 

 underlying particles or objects. 



One plan was to patch on a right-angled prism by its long side 

 with water, to the under surlace of the slide, and then condense the 

 Light through either of the faces by lens adaptations. Another 

 arrangement was by means of a solid glass parabola with a flat top, 

 on which the slide was laid with water. This is now revived under 

 the name of the " immersion paralx3loid." The one that I then made 

 I possess still, and employ occasionally. But the plan then pre- 

 ferred and in use now. without the slis^htest modification, is the 



1 • • • 



combmation of the ordinary parabola and the tnincated lens, as tbis 

 gave far more hght than any of the others. The method has not 

 been much used, as the number of objects in fluid or balsam capable 

 of intercepting light as strictly opaque objects were remarkably 



VOL. VII. s 



