In a “Preliminary Note on the Reproduction of Diffraction-gratings by means of Photography,” published in the Proceedings of the Royal Society for 1872, and in the Philosophical Magazine for November of the same year [Art. XVII], I gave a short account of experiments with which I had been for some time occupied. A few further details were communicated to the British Association at Brighton (Brit. Assoc. Report, p. 39) [Art. xvm]. I now propose to give the results of more recent experience in the practical manufacture of gratings, as well as some theoretical conclusions which have been in manuscript since the subject first engaged my attention.

There are two distinct methods of copying practised by the photographer —(1) by means of the camera, (2) by contact-printing. The first, if it were practicable for our purpose, would have the advantage of leaving the scale arbitrary, so that copies of varying degrees of fineness might be taken from the same original. By this method I have obtained a photograph of a piece of striped stuff on such a scale that there was room for about 200 lines in front of the pupil of the eye, capable of showing lateral images of a candle; but I soon found that the inherent imperfections of our optical appliances, if not the laws of light themselves, interposed an almost insuperable obstacle to obtaining adequate results.

However perfect a lens may be, there is a limit to its powers of condensing light into a point. Even if the source from which the light proceeds be infinitely small, the image still consists of a spot of finite size surrounded by dark and bright rings.