72 THE MICROSCOPE. 



something about before owning, is the objective, and to understand 

 this subject in its outlines, we must review, briefly, the nature of 

 the things to be seen by the objective, and the powers which the 

 objective must have in order to make these objects visible. 



Objects which are visible to the eye present to the mind the 

 image of the object as a whole, and more or less of the structural 

 elements or factors of the object. We see many objects as a whole 

 every day, the small parts of which, or the anatomical elements or 

 factors of which, we cannot see, because they do not come within 

 the angle of vision of the eye. We can see the images of small ani- 

 mals perhaps, whose ears and eyes, and perhaps other members are 

 not visible. 



Now, the same laws, of course, hold good in microscopic ob- 

 jects. An object, as a whole, may be too small to be seen by the 

 eye — for instance, a diatom, or blood globule, but a lens which am- 

 plifies the object five or fifty diameters, will enlarge the apparent 

 size of the object, by its amplifying power, and will make visible 

 more or less of its anatomical or structural factors, by reason of its 

 angular aperture, as well as its focal distance, or refractive power. 



There are two factors then in the power of objectives which aid 

 vision, and in selecting an objective both of these factors must be 

 considered. The distinction to be borne in mind is that the focal 

 distance has relations of particular character to amplification of the 

 general image, and the angular aperture has more special relations 

 to the resolution of the structural elements of objects which are 

 looked at through a microscope. 



An objective must have enough angular aperture to delineate 

 or resolve the smallest elements of any general image, so as to ad- 

 mit of their amplification, and the objective (with eye pieces) must 

 have refractive power enough (focal distance) to amplify the general 

 image and images of the structural elements, so as to form a visual 

 angle. 



Just what a visual angle is, I don't know, but it is said to repre- 

 sent the distance between two contiguous rods or cones in the fovea 

 centralis. 



I have said that the average eye can see lines ruled on glass 

 about 200 to the inch, or the eye can see structural elements in any 

 object which have a size of -phs °f an inch. Now any object, big 

 or little, which can be seen by the eye, if it is made up of structural* 



