470 Miscellaneous. 



senting every object without colour and without spherical aberra- 

 tion ; when the microscopic object is a cube, we shall see five of 

 its faces, and when it is a sphere or a cylinder, we shall see nine- 

 tenths or more of its circumference. How then does it happen 

 that large apertures exhibit objects which are not seen when 

 small apertures with the same focal length are employed ? This 

 superiority is particularly shown with test objects marked with 

 grooves or ridges and obliquely illuminated. The marginal part 

 of the lens will enlarge the grooves and ridges, and they will 

 thus be rendered visible, not because they are seen more distinctly, 

 but because they are expanded by the combination of their inco- 

 incident images. Hence we have an explanation of the fact — 

 well known to all who use the microscope, — that objects are seen 

 more distinctly with object-glasses of small angular aperture. In 

 the one case we have, with the same magnifying power, not only 

 an enlarged and indistinct image of objects, but a false represen- 

 tation of them, from which their true structure cannot be discov- 

 ered ; while in the other we have a smaller and distinct image, 

 and a more correct representation of the object. But these are 

 not the only objections to large angular apertures and short focal 

 lengths. 1. In the first place, it is extremely diflacult to illumi- 

 nate objects when so close to the object-glass. 2. There is a 

 great loss of light, from its oblique incidence on the surface of 

 the first lens. 3. The surface of glass, — with the most perfect 

 pohsh, — must be covered with minute pores, produced by the 

 attrition of the polishing powder ; and light, falling upon the 

 sides of these pores with extreme obliquity, must not only suSer 

 difi'raction, but be refracted less perfectly than when incident at a 

 less angle. 4. "When the object is almost in contact with the 

 anterior lens, the microscope is wholly unfit for researches in 

 which mechanical operations are required, and also for the exa- 

 minations of objects inclosed in minerals or other transparent 

 bodies. 5. In object-glasses now in use, the rays of light must 

 pass through a great thickness of glass of doubtful homogeneity. 

 It is a question yet to be solved whether or not a substance can 

 be truly transparent, in which the elements are not united in 

 definite proportion ; in which the substances combined have very 

 difl"erent refractive and dispersive powers ; and in which the par- 

 ticles are so loosely united that they separate from one another, 

 as in the various kinds of decomposition to which glass is liable. 

 If the best microscopes are efiected by these sources of error, 

 every exertion should be made to diminish or remove them. 



