480 Grubb — On the Construction of Telescopic Object- Glasses. 



strongly positive. Sir George Stokes' suggestion is to grind the crown rather more 

 convex on the inside than the outside, leaving the power of both crown and flint as 

 before. The result of this will be to introduce a little positive spherical aberration. 

 Let that be corrected by figuring the lens. When required for photographic use 

 the lenses are separated as far as necessary to correct for photograjihic rays, and 

 the crown lens is then reversed. The crown is now in a position of lower 

 (positive) spherical aberration, and this balances the amount of aberration intro- 

 duced by the separation. This has been successfully tried with small-sized 

 objectives. 



