GHOSTS AND OCULARS. 53 



the index of refraction, and/1,/2, etc. the focal lengths of the equivalent 

 thin lenses obtained in the ordinary way. It hence is obvious that 

 the radius of curvature of the sharp image in a single crown glass lens 

 is practically f / while in a 2-lens combination widely separated, 

 as in the common forms, it is about -|/. Other oculars lie between 

 these limits. Evidently highly refracting glasses relieve the situation 

 a little, but they are rarely used, being rather harder to get than com- 

 mon crown. Since the easy range of accommodation of the eye except 

 in early youth is not much over 2 diopters the extent to which it can 

 compensate the curvature of the image is rather unsatisfactory, except 

 in low power eyepieces. Conrady (M. N. 78, 445) in discussing this 

 subject has shown that for a total field of 40° the sharp field fails in 

 ordinary eyepieces for focal lengths under an inch, while a carefully 

 designed achromatic combination, e.g., i^4, 5, 9, Figure 3, will reduce 

 this figure to about ^ in., and he further shows a very interesting possi- 

 bility in the construction of anastigmatic eyepieces to give at once a 

 field at once flat and free from marginal astigmatism. It is to be hoped 

 that this masterly optical investigator will carry out his own sugges- 

 tions, but even without anastigmats it is quite feasible to get greatly 

 improved eyepieces of moderately large field and with interference 

 from reflections greatly reduced. 



The freest of all from reflections are, of course, the single lenses, 

 of which the more common forms are shown in numbers 1 to 5 of 

 Figure 3. The simplest of them is the ordinary plano-convex lens 

 such as is obtained by removing the field lens of a Huyghenian ocular. 

 It gives somewhat increased light but a very small sharp aperture, 

 not over 10°. A little better field and similar illumination is given 

 by the solid eyepiece commonly know^n under the name of Codding- 

 ton, although due to Sir David Brewster. It is derived from a glass 

 sphere by removing a thick equatorial belt and then cutting an equa- 

 torial groove for a stop, down to a diameter of something less than 

 half the radius of the sphere. Such an ocular of crown glass is of 

 focus about three halves the radius of the sphere and the field is rather 

 larger than that of a simple lens, but badly chromatic toward the edges. 



The next step in improvement of results from the single lens is 

 the simple achromat, ^ 3, such as is made for an eyepiece by Cooke 

 and others. When well designed d:nd preferably of high index glasses 

 it gives a field of from 15° to as high as 20°, free of color, but not fully 

 orthoscopic. Next, one comes to the triple achromats, of which # 4 

 is a typical example, as made by Zeiss, Steinheil, and others. The 

 field of such a triplet is usually good for 20° to 30°, colorless, quite flat 



