THE ULTRA-MICROSCOPE 155 



microscope, however perfect the lenses are and however 

 great the magnification is. This, in fact, is the limit of 

 microscopic resolution. It is true that with an apparatus 

 known as the ultra-microscope, which consists essentially 

 in viewing on a dark background the objects illuminated 

 with a powerful transverse beam of light, it is possible to 

 render visible particles of dimensions far below this, but 

 the particles appear merely as points of light no form 

 or structure is visible. What has happened is that the 

 light striking the particles is " scattered " or diffracted, 

 and the area of the scattered light, provided the beam of 

 light be sufficiently powerful, is large enough to be 

 visible. 



Lenses or objectives are of two kinds, achromatic and 

 apochromatic. In the achromatic objective, the con- 

 stituent lenses are constructed by cementing together 

 elementary lenses of crown and of flint glass in order to 

 correct for spherical and for chromatic aberration. The 

 rays passing through the centre and the margin of a 

 simple lens are not focussed at the same point ; this is 

 spherical aberration. If uncorrected, it results in a 

 blurred image ; it could be obviated to some extent by 

 " stopping out " the marginal rays with a diaphragm, 

 but this procedure diminishes the angle of aperture and, 

 therefore, the N.A. and resolving power. Similarly, the 

 rays at the red and violet ends of the spectrum are of 

 different refrangibility, and, a simple lens acting like a 

 prism, coloured fringes are observed ; this is chromatic 

 aberration. By suitable combinations of lenses of crown 

 and of flint glass, both these defects of a simple lens can 

 be to a large extent corrected. In the apochromatic 

 system of lenses these defects are still more perfectly 

 corrected by the use of special glasses with fluorite, cor- 

 rection being partly effected in the objective and completed 

 in the eye-piece or ocular. With apochromatic lenses, 



