138 TRANSACTIONS OF THE 



even monocliromatic liglit. The Kellner eye-piece C, if placed in 

 the sub-stage and provided with a number of stops fitting over the 

 eye-lens instead of the cap, makes a very good condenser and 

 yields abundant light up to 10,000 diameter magnification. 



The "Webster and Swift condensers are each of them a multum in 

 parvo, uniting in an efficient and not expensive way all the various 

 pieces of sub-stage illumination apparatus, and will be found quite 

 satisfactory in nearly all cases. I have obtained excellent results 

 with the paraboloid for light ground illumination under the highest 

 powers, as it gives plenty of oblique and white light. In fact I 

 have, by its aid, resolved certain markings which some first-class 

 achromatic condensers refused to bring out. 



The diffusion condenser, invented by Mr. Wenham, gives 

 sj^lendid efiects on transparent objects viewed under the stereoscopic 

 binocular microscope. It consists of an achromatic combination of 

 170°, having a white cloud cap, i.e., a cap of two thin glass discs 

 between which some powdered glass is placed, which breaks up the 

 pencils of light in all directions ; it yields a soft white light, and 

 by its means the two fields of the binocular are easily illuminated. 

 This is sometimes a difficulty under medium powers, which may 

 be overcome by placing a piece of roughened glass under the 

 object. The object should be well lit up, but not too much so, else 

 the binocular efiect will be destroyed. Beginners generally think 

 that the best efiects are obtained by fiooding the field with light, 

 which is a very great mistake. 



The next class of condensers are those for dark ground illumin- 

 atio7i of transjxirent objects, the simplest is the spot-lens. This is 

 a large plano-convex or bi-convex lens, the upper and lower surfaces 

 of which with the exception of a narrow border are ground flat 

 and blackened, so that if the light from the mirror is thrown on 

 it, the marginal rays alone pass through it, and these being much 

 refracted pass over the object without entering the object-glass; 

 the result is brilliant illumination of the objects on a black ground. 

 The spot-lens is suitable for object-glasses up to half- inch focus. 

 A more perfect instrument for the same purpose is the Paraboloid 

 (Fig. 16) of Mr. Wenham. A parabola is a curve produced by 

 the section of a cone and of a plane under certain conditions, and 

 it has the property of reflecting all rays, which fall on it parallel 

 to its axis, to one point in the same, called the focus. By rotating 

 the parabola round its axis, we get a paraboloid which has the 



