If now the field lens of the ocular be made of heavy flint glass and the separation of the lenses suitably adjusted, the stronger refraction of the field lens for the blue pulls up the blue focus and brings its image to substantially the dimensions of the red, so that the eye lens performs as if there were no overcorrection of the objective. The writer has experimented with an ocular of this sort as shown in Fig. 102_b_ and finds that the color correction is, as might be expected, greatly improved over a Mittenzwey ocular of the same focus (⅕ inch). There would be material advantage in thus varying the ocular color correction to suit the power. In the Huyghenian eyepiece the equivalent focal length F is given by, F = 2ff′/(f + f′) where f and f′ are the focal lengths of the field and eye lenses respectively. This assumes the normal spacing, d, of half the sum of the focal lengths, not always adhered to by constructors. The perfectly general case, as for any two combined lenses is, F = ff_{1}/(f + f_{1}-d) [Illustration: 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. The two neighboring rays _1_, _2_, coming through the objective from the distant object meet at the objective focus in a point, _a_, of the image plane _a b_. Thence, diverging, they are so refracted by _A_ and _B_ as to leave the latter substantially parallel so that both appear to proceed from the point c, of the image plane _c_, _d_, in the principal focus of _B_. From the ordinary equation for the combination, F = ¾ f. The combination focusses ¼ f back of the principal focus of the objective, and the position of the eye is ¼ F back of the eye lens, which is another reason for shortening the lens spacing. At longer spacing the eye distance is inconveniently reduced.