Highly Magnified Images. By J. W. Gordon. 



21 



result of diffraction from a wave-front, will in its turn give rise 

 by further diffraction to a inonophasal surface or true wave-front, 

 a wave-front which, in the case supposed, would have a conical 

 form. It will be interesting to consider what are the phenomena 

 to which such a conical wave-front would give rise. 



Let P of fig. 13 be such a conical wave-front, and suppose, 

 first, that the observer's eye is focussed on the plane p, situated a 



Fig. 13. 



Fig. 14. 



little below the apex of the cone. It is clear that the conical 

 wave-front, optically projected backward on to the focal plane, will 

 in that case produce a nebulous light in the middle of the field 

 fading off towards the outer edge, where the illuminated zones are 

 largest, and forming a strongly marked boundary at the inner 

 edge, where a dark spot occupies the actual centre of the field. 

 It may be observed that this nebula is, in fact, an expanded 

 antipoint. Fig. 14 is actually drawn from such a nebula with the 

 black dot at its centre. 



Next suppose the focal plane to be carried up to the position 

 p 2 shown in fig. 15. The part of the cone above the focal plane 

 will be optically projected down upon it, and the part below 

 will be propagated up to it, with the result that the nebula will 



pz 



Fig. 15. 



Fig. 16. 



be diminished in extent, enhanced in brightness, and its centre 

 occupied by a strong point of concentrated light. The black 

 dot has changed into a white dot. Fig. 16 is a drawing of a white 

 dot produced in this way. 



Finally, assume the focal plane to be carried up to the 

 position jh of fig- 17. Here the nebula becomes still larger, 

 and, being more diffused, weaker towards its outer edge. The 

 inner edge will, however, still be strong, and will clearly mark off 

 ■a black dot in the centre, so that we have returned, not indeed 

 in the structure of the image, but almost exactly in point of 

 appearance, to the condition of things illustrated by fig. 14. 



