THE MICROSCOPE AND ITS ACCESSORIES. .13 



linen cloth moistened with benzene may be used to remove the oil 

 from the lens. After use, the cover-glass must be wiped dry, as 

 well as the lens, by means of a clean piece of wash-leather. These 

 lenses are very good, but very dear, and are not required for ordinary 

 work. Latterly, instead of oil, a mixture of glycerine and chloral 

 hydrate has been recommended. The cedar-oil is not so easily 

 removed from the cover-glass. It may be best removed by a well- 

 washed linen rag moistened with benzene. The lens itself must be 

 most carefully cleaned. A lens moistened with glycerine is best 

 cleaned with alcohol. 



12. Angle of Aperture. Lenses may be of narrow or wide angle 

 of aperture. The angle of aperture is the angle formed by the 

 outermost rays coming from a luminous point placed in the focus of 

 an object, and which enter not only the lowest lens of the objective, 

 but pass throughout the entire system of the lenses of the objective. 

 Lenses with a large angle of aperture (130 and upwards), therefore, 

 will admit more light, i.e., more of the oblique rays will enter the 

 system of lenses. These lenses are well suited for resolving line 

 lines on the surface of an object, such as the striae on the scales of 

 insects' wings and the markings on diatoms, but those parts of the 

 object superficial to or deeper than the focus are not sharply defined. 

 Such a lens is said to have greater power of resolution. For ordi- 

 nary histological work, a high-power (350-400) lens of medium 

 (8o-ioo) angular aperture is to be preferred, for such a lens has 

 greater penetrating power, i.e., the focal plane is deeper, so that with 

 it one can see with tolerable distinctness parts of the object lying 

 immediately above and below the true focus of the lens. It is to 

 be remembered that the angle of aperture has nothing to do with 

 the magnifying power of the lens. 



13. Abbe's Apochromatic Lenses. These are dry or immersion, 

 and are used as high-power lenses when very exact definition is 

 required. They are very expensive, and are constructed of a 

 peculiar kind of glass. These objectives secure the union of three 

 different colours of the spectrum in one point of the axis, i.e., they 

 remove the so-called "secondary spectrum," and they correct the 

 spherical aberration for two different colours. The images projected 

 by them are nearly equally sharp with all the colours of the 

 spectrum. As there is a very great concentration of light by these 

 objectives, they permit of the use of very high eye-pieces, thus giving 

 high magnifying power with relatively long focal length. The natural 

 colours of objects are reproduced unaltered by these objectives. 



Zeiss has constructed a series of compensating oculars to be 

 used with these lenses. They are classified as i, 2, 4, 8, 12, 18, 

 and 27, according to their magnifying power. The eye-pieces of 

 extremely low power are designated 



