F = ff1/(f + f1-d)
Fig. 103.—Path of Rays Through Ramsden Ocular.
To obtain a flatter field, and particularly one free from distortion the construction devised by Ramsden is commonly used. This consists, Fig. 103, of two plano convex lenses of equal focal length, placed with their plane faces outward, at a distance equal to, or somewhat less than, their common focal length. The former spacing is the one which gives the best achromatic compensation since as before the condition for achromatism is
d = ½(f + f′)
When thus spaced the plane surface of the field lens is exactly in the focus of the eye lens, the combined focus F is the same as that of either lens, since as just shown in any additive combination of two lenses
F = ff′/(f + f′-d)
and while the field is flat and colorless, every speck of dust on the field lens is offensively in view.
It is therefore usual to make this ocular in the form suggested by Airy, in which something of the achromatic correction is sacrificed to obviate this difficulty, and to obtain a better balance of the residual aberrations. The path of the rays is shown in Fig. 103. The lenses A and B are of the same focal length but are now spaced at ⅔ of this length apart.