﻿1000 



Mr. W. N. Bond 



on a 



The lens in its original form consists of a glass hemi- 

 sphere L (fig. 1) of radius r. The light is incident on the 

 plane face, which is covered by a thin screen S. except for 

 a small circular aperture at the centre. It will be seen 

 that a ray incident in the plane of the outer face of the 

 lens will be refracted at the critical angle. Furthermore, 

 all refracted rays will arrive almost normally at the hemi- 

 spherical face of the lens, and will subsequently converge 

 to form an image, which, for objects at infinity, will lie on 

 part of a sphere III, concentric with the hemispherical face 

 of the lens and of radius r/x/(/jb — l), where //, is the refrac- 

 tive index of the lens. The emergent cone of rays will in 

 practice have a total angle of slightly less than 90° ; and if 



the aperture in the screen S is small enough, the whole 

 image will be sufficiently in focus on a flat plate PP placed 

 at a distance from the plane face of the lens equal to the 

 mean distance of the various portions of the true image III 

 from this plane face. 



The screen S should be covered on the outer side by a 

 plane plate of glass G, the whole being cemented together. 

 This arrangement avoids the finite thickness of the screen S, 

 preventing some or all of the light incident at fairly oblique 

 angles from entering the lens. 



It will be seen that the photographic plate should be 

 placed at a distance h from the plane face of the lens of 

 about 2'5 r (i.e. rather less than fjur/{fjL — l)). It is easy 



