METHOD OF VIEWING.
The pictures obtained are colourless and almost imperceptible when looked at in the ordinary way. In order for the colours to be seen, they must be lit in a particular fashion and examined through a small aperture, so placed that the eye is in a correct position for viewing. The apparatus devised for the purpose by Pro fessor Wood is shown by Fig. 583. It consists of a stand carrying at one end a metal plate with two eyeholes, and at the other a support to take the diffrac tion photograph, behind which is fixed a double-convex lens. The necessary illumination must be obtained from a narrow slit parallel with the lines of the gratings. An incandescent mantle at some distance from the picture will give satisfactory results, or the filament of an electric lamp placed sideways is very suit able. At first, these pictures were arranged to be viewed with one eye only, a manifest disadvantage ; but later it occurred to the inventor that as two spectra were formed by the grating, one on each side of the central line, it should be possible to use both eyes at once if the two openings were placed so that one eye should see the first spectrum and one the other, which is now clone. A suggestion clue to Mr. F. E. Ives is that the lines of the gratings should be used horizontally instead of vertically, when the pictures could be inspected through a horizontal slit, and the eyes might be moved in any direction along the line without the colours appearing different.
This plan would have the advantage of being suited to any pair of eyes, for it is a well-known fact that these are not always the same distance apart in various individuals. Professor Wood has now succeeded in modifying the apparatus so that two gratings can be viewed at once, thus enabling the pictures to be seen stereoscopically.
1iEPRODLCTION OF THE POSITIVES.
Any number of copies of a diffraction photograph may be obtained by printing in contact on a film of bichromated gela tine. A curious feature of the process is that a positive picture is always secured from a positive by one operation, no negative being required ; since the lines and spaces of the original are re produced at similar distances, and that is all that is necessary. The image, although scarcely visible, and certainly not possessing anything of what is known as photographic density or gradation, s nevertheless copied faithfully as re ards all its rulings in microscopic furrows and ridges of gelatine, by means of that very diffraction of light which enables the finished picture to be seen in the colours of nature. This ready repro duction of diffraction photographs is mani festly a decided and unique advantage.