WENHAM, ON THE BINOCULAR MICROSCOPE. Ill 



would earnest^ recommend the makers to construct the set- 

 tings of the highest powers in future as short as they safely 

 can_, in order to obviate this want of field as far as possible. 

 I can offer no other suggestion_, and the remedy is solely in 

 their hands. 



I have before stated that the employment of a strong 

 direct light should be avoided for 

 the illumination of objects ob- 

 served with the binocular micro- 

 scope_, as direct raj^s tend to de- 

 stroy the stereoscopic effect. For 

 this reason I recommended the 

 use of diffused light. The an- 

 nexed figure shows a form of 

 illuminator that has been found 

 to give an excellent effect. It 



consists of three plano-convex lenses of the diameters 

 and radii shown; it condenses a very large area of light, 

 and consequently gives great intensity. The final emer- 

 gent pencil has an angular aperture of 170°. Just above 

 the top lens of the combination there is a sliding-cap, 

 the crown of which contains the diffusing film. For this I 

 have had some difficulty in finding a perfectly white and 

 homogeneous^ and at the same time partly transparent, 

 material. What I now employ is the beautiful snow-white 

 powder obtained from turning glass with a diamond turn- 

 ing tool. This may be procm'cd from the opticians, and 

 should be well washed, to free it from the larger particles. 

 A thin film of this impalpable powder is then compressed 

 between two discs of thin glass, and fixed in the top of the 

 sliding-cap, which is to be raised or lowered till the most 

 intense light is obtained on the film. This illuminator 

 is employed in the position of the achromatic condenser. I 

 generally place a disc of slightly coloured neutral tint glass 

 below the bottom lens, as it increases the purity of the light, 

 and gives greater distinctness to objects. The effect of this 

 difiusing film is sometimes enhanced by condensing light 

 down on the object ft'om above as well as below. In fact, 

 in the use of the binocular microscope, I am constantly in 

 the habit of placing the light so as to illuminate both as a 

 transparent and opaque object at the same time, so that each 

 method is ready to be used separately or together as may be 

 found requisite. 



