Glasthule Lodge, Kingstown, Ireland,

August 16, 1886.

A happy thought occurred to me to-night as to a mode of adapting the Greenwich Refractor that is to be put to photography. I hope it may solve the difficulty. It is to make the crown-glass, lens of the objective extremely nearly, but not quite, equi-convex, and to mount it in a cell which could be put either way in, the difference of radii of the two surfaces being made to accord with a calculated difference.

For vision, the flatter surface would be turned outwards, and the lenses placed nearly in contact. For photography, the lens would be reversed, and its distance from the flint-glass would be greater than before by a known quantity which would reduce the chromatic correcting power of the flint to what is required for photography.

The separation alone would make the spherical aberration positive, and the reversion alone would make it negative, and if the small difference of radii has the right value these two will compensate each other. Thus the spherical aberration will remain at zero while the chromatic compensation is altered from that suited for vision to that suited for photography.

The focal length of the objective will of course be shortened a little by the separation, but not I think to an inconvenient degree.