GHOSTS AND OCULARS. 

 By Louis Bell. 



Received October 13, 1920. PreseateJ October 13, 1920. 



The visibility of celestial objects, as of objects in general, depends 

 on brightness contrast with the background. The more general light 

 in the field of vision the more must come from a particular object to 

 bring it within range of the eye. The average total brightness of the 

 sky has been shown by Yntema (Gron. Pub. 7^ 22) and van Rhijn (Ap. 

 J. 50, 374) to be 0.13 or 0.14 of a 1st magnitude star per square degree. 

 Considering the naked eye as an optical instrument of about 5 mm. 

 aperture, one finds equality of the image with this background at 

 about 6™. 5, which suggests that the contrast required for visibility is 

 fully 0'"..5. With telescopes the case is less favorable since 35 to 40 

 per cent, of the light is generally lost in objective and ocidar, substan- 

 tially half in the latter. The loss by reflection goes chiefly outside 

 the instrument as regards the objective. The loss in the ocular is 

 chiefly scattered on diaphragms and other surfaces close to the eye 

 thus lighting up the field, and part actually passes directly to the eye. 

 The result is a material loss of visibility in faint objects due to the 

 enhanced brightness of the field, and sometimes the appearance of 

 false images. 



This investigation is directed at the magnitude and distribution of 

 the reflected components of the light and particularly those wliich are 

 within the ocular, too often constructed without regard to the effect 

 of its reflections on the field viewed by it. 



For the present purpose a ghost in an optical instrument may be 

 defined as an image, real or virtual, formed by the reflection of light 

 from the surfaces of the system. It is commonly far out of focus so 

 that it merely forms a hazy spot in the field, or diverging still more 

 widely merely serves to scatter a small amount of light over the whole 

 field. Under certain conditions it may, so to speak, materialize as a 

 fairly bright image, in which case it becomes a nuisance. Most 

 visible ghosts are referable to the ocular of the instrument, whence 

 the desirability of considering the ocular, which has been grossly 

 neglected. 



Any beam of light incident normally on a lens system suffers reflec- 

 tion at each refracting surface according to the perfectly well known 



