52 BELL. 



prism surface lies just ahead of the flat face of the field lens with three 

 other optically parallel faces in reserve. Three ghosts are conspicu- 

 ously bright. One, the ordinary field-lens ghost, one from the reflec- 

 tion between the front of field lens and prism face, and the third by 

 reflection from back of field lens and prism face. Besides these at 

 least two others rather faint and dodging quickly about in the field 

 can be made out, chargeable to the adjacent rear prism faces. 



With respect to oculars themselves the general rule applies regard- 

 ing the number and effect of reflecting surfaces, with the same proviso 

 that the 2nd order ghosts are the ones seen, while the 1st order may 

 cause troublesome reflections from the mount. 



The fact is that oculars have received very scant treatment at the 

 hands of practical opticians despite the fact that half the light lost 

 in the telescope is generally lost in the ocular, and that it is the seat 

 of no inconsiderable trouble from stray light. It is not the loss of 

 light m the image that hurts, but the effect of the light lost. What is 

 the use of an objecti^•e of the highest corrections producing a perfect 

 primary image if that image is magnified by an eye piece of indifferent 

 definition and appears shrouded in a haze of scattered light ? 



The ordinary ocular has a strongly curved field, somewhat distorted 

 and imperfectly corrected for color. Despite the fact that very 

 material improvements have been made, mostly to meet the severe 

 requirements of microscopic vision, telescope makers stick quite 

 closely to the old Huyghenian and Ramsden forms, chiefly perhaps 

 because these are composed of simple plano-convex lenses and are 

 therefore easy and cheap to make. There is a tendency, too, toward 

 struggling for an abnormally wide field at the expense of definition. 

 Nearly all eye pieces, and particularly these mentioned, have a not- 

 ably curved field or else are viciously astigmatic in the outer zone, 

 and these faults increase with the angle of view. The eye can take 

 in above 40° but not much more than half this angle is anywhere nearly 

 simultaneously sharp and anything materially over 40° is only attained 

 by peering around the edges of the field. 



Fortunately the only cases in which wide field is really useful are 

 those in which the magnifying power is relatively low, which enables 

 the accommodation of the eye to help out. The curvature of the image 

 derived from Petzval's equation is 



1111 1 1 _ 1 



— - — = —r + — : + etc. = ;— . ; when Ro is large, ^ = 2- -^• 



Where Ro is the radius of curvature of object, Ri of image, n, as usual. 



