244 Mr. Airy on Achromatic Eye-pieces of Telescopes, 
rays, will prevent the object from being distinctly seen: and 
because it is impossible to apply a micrometer. Add _ to this, 
that the eye must be in contact with the lens CF, or it will not 
receive all the pencils from different points of the object. 
(14). If a be greater than g, the image is formed between 
the two lenses; and, since there is always some distortion at 
refraction by a single lens, the micrometer cannot safely be 
applied. Ifa be less than q, we shall have, (since g=2a—-p) 
a~F>p, or a> pb > BG. 
In this case the pencils cross before they can be received by 
the eye: this construction therefore cannot be employed im any 
case. The achromatic eye-piece of two glasses can never there- 
fore be used with a micrometer. 
(15). In telescopes with a micrometer, the eye-piece generally 
consists of two lenses, and the image is formed between EB 
and the object-glass. As no part but the center of the field 
of view can be distinctly seen, there is an apparatus which 
enables the observer to slide the eye-piece across the end of 
the telescope, and thus move the center of the field to the 
object. This kind of eye-piece is called by artists the positive 
eye-piece in contradistinction to that in which the image is 
formed between the eye-glasses, which is called the negative 
eye-piece. With the positive eye-piece the lens EB should 
always be placed as near the first image, and the lens CF as 
near the intersection of the pencils with the axis, as convenience 
will allow. , 
(16). On the eye-piece with three eye-glasses. Let p, g, 7, 
be the focal lengths of the lenses, beginning with that nearest 
to the object-glass; let a ‘and b be their distances: and for 
simplicity, suppose the object-glass so distant, that the axes of 
