752 



SILVANUS P. THOMPSON. 



theii covered with a zone-plate, prepared by painting a nnmber of con- 

 centre rings in opaque black varnish upon a dise of glass which conld 

 be attached to the lens. Tins at once had the. effect of dividing the 

 coma in the manner indicated by the internai lines shown in the Figures. 

 The middle portion of the lens j^roduced as its contribution to the total 

 image the brightest and most concentrated part near the narrow end of 

 the coma, whilst the outermost zone of the lens contributed the most 

 distorted and outermost rim of the balloon-shaped periphery. 



Experiment 2. The arc lamp was used without any condenser. The 

 same lens, provided with a zone-plate, was set as described at about35° 

 to the pencil of light about 90 centimètres from the lens. The vertical 

 transparent screen already mentioned was then placed in close juxtapo- 

 sition to the lens, so that it almost touched the upper edge of it. In 

 this position, far within the principal focus, the forms of the opaque 

 annuli of the zone plate were cast, distorted, upon the screen, with the 

 intervening annulai* patches of light betweenthem as transmitted through 

 the several zones of the lens. The forms of thèse distorted annuli were 

 then traced in outline on the screen, or rather on a pièce of tracing 

 paper laid upon it. The screen was then moved back three centimètres, 

 the annuli now showing a différent distortion, and in this position a 

 second tracing was taken. Again the screen was moved back another 

 eight centimètres, and a third tracing obtained. This was repeated at 

 regular distances and a séries of 10 pictures obtained extending to a 

 point in space a little beyond the principal focus of the (non-tilted) 

 lens. Thèse 10 pictures are reproduced in the annexed plate, Fig. 4. 



On examining this séries of pictures it becomes easy to follow out the 

 râle played by each of the several zones of the lens. If the lens had 

 been set to receive the light normally, instead of obliquely, the light 

 from any one zone cast upon a screen placed [as was the case in Fig. 4 

 (1),] near the lens would have depicted simply an annulus; and as the 

 screen was moved away from the lens this annulus of light would have 

 contracted regularly down to a point at the focus of that zone, and 

 would then have again enlarged regularly as the screen was still further 

 removed. But as the lens is tilted, the annulus ismuch distorted. Inspec- 

 tion of Fig. 2 will show that the light which traverses the lower part 

 of the outer zone suffers a much greater amount of refraction than that 

 which traverses the upper part of the same zone. Consequently the 



