Finally, we come to the solid eyepiece Fig. 102a, devised by the late R. B. Tolies nearly three quarters of a century ago, and and often made by him both for telescopes and microscopes. It is practically a Huygenian eyepiece made out of a single cylinder of glass with a curvature ratio of 1½:1 between the eye and the field lens. A groove is cut around the long lens at about ⅓ its length from the vertex of the field end. This serves as a stop, reducing the diameter of the lens to about one-half its focal length.
It is in fact a Huygenian eyepiece free from the loss of light in the usual construction. It gives a wide field, more extensive than in the ordinary form, with exquisite definition. It is really a most admirable form of eyepiece which should be used far more than is now the case. The late Dr. Brashear is on record as believing that all negative eyepieces less than ¾ inch focus should be made in this form.
Fig. 102.—Tolles’ Solid and Compensated Oculars.
So far as the writer can ascertain the only reason that it is not more used is that it is somewhat more difficult to construct than the two lens form, for its curvatures and length must be very accurately adjusted. It is consequently unpopular with the constructing optician in spite of its conspicuous merits. It gives no ghosts, and the faint reflection at the eye end is widely spread so that if the exterior of the cylinder is well blackened, as it should be, it gives exceptional freedom from stray light. Still another variety of the Huygenian ocular sometimes useful is analogous to the compensating eyepiece used in microscopy. If, as commonly is the case, a telescope objective is over-corrected for color to correct for the chromatism of the eye in low powers, the high powers show strong over correction, the blue focus being longer than the red, and the blue image therefore the larger.
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. 102b 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,