B4 



refracts tlie marginal or more oblique rays in a greater ratio than 

 the central ones. 



The rays of light, after leaving the second surface of the glass, 

 proceed onwards, parallel to their original direction, and intersect 

 the axis at various distances, producing a longitudinal aberration, 

 nearly equal to the thickness of the glass, the point of the marginal 

 or outstanding rays being the most distant. 



The effect of a flat, parallel plate of glass upon a pencil of rays is 

 that of over correction, or, in other words, the aberrations, both 

 chromatic and spherical, are contrary to those produced by a convex 

 lens, in which the marginal rays are brought to a focus nearest to 

 the lens, while the central ones are thrown to a more distant point. 



It appears from this that if a lens of proper focus and curvature 

 be placed at the apex of the reflector, it will neutralize both the 

 chromatic and spherical errors caused by a certain thickness of 

 glass under the object. This result has been obtained, as shown by 

 fig. 5 (drawn four times the full size). AA is a meniscus, of 1'4 in. 

 focus; the radii of the surfaces are as 4' to 3'125. This is shown 

 in the diagram, in such a position that the centre of the convex 

 surface of the meniscus is in the focus, B, of the parabola, therefore 

 the rays of reflected light, CC, proceeding towards the point, B, 

 enter the under convex surface of the meniscus radially, without 

 suffering any refraction ; and after meeting the concave surface, and 

 emerging therefrom, the outstanding rays intersect the axis at D, 

 while the inner ones are thrown more distantly, as at E, producing 

 a longitudinal aberration from D to E, as shown by the dotted lines, 

 thus giving a sufficient amount of under correction for glass a 

 twentieth of an inch thick, for it is shown that by interposing a plate 

 of glass, FF, of that thickness, the focus of the marginal rays is 

 brought up to that of the inner ones, so that the whole pencil of 

 rays comes to a point through the glass. 



Fig. 6 is given for the purpose of showing that there is a means 

 of adjustment for all thicknesses of glass under a twentieth of an 

 inch. B is the focus of the parabola. The dotted outline represents 

 the position of the meniscus, as shown in fig. 5. AA is the meniscus 

 lowered from its former position. The centre of curvature of the 

 convex surface of the meniscus now being under the focus of the 

 parabola, the rays enter that surface at an angle, or otherwise than 

 radially, and after being refracted by both surfaces of the meniscus 

 are brought to a focus. It is then suitable for an object without 

 any glass underneath it. 



