It is now possible for the traveller to bring home records of what he has seen in natural colours. There are two or three known methods by which this may be done, but few are available for the work of exploration. The picture obtained by the method patented by Mr. Ives and named Krōmskōp Photography is produced by three monochrome images. These have however been taken through three tinted glasses in a camera of a special kind. The images may be thrown upon a screen by means of a special lantern; the light passes through tinted glasses of colours complementary to those employed in making the negatives with the result that the picture on the screen exhibits all the varied hues of nature. The devices however are exceedingly expensive and can be satisfactorily employed only by those who thoroughly understand the apparatus and the problems which have to be solved. Simpler processes are the Autochromic and The Paget Colour Plate process. In the Autochromic process the manufacturers coat a sheet of glass with minute specks of three colours, blue, violet green and orange, irregularly spread, and lay a coat of panchromatic emulsion on top. The photographer exposes through the glass, thus obtaining a negative. The developed silver, the negative image, is dissolved away and the silver bromide remaining is developed giving a positive image in natural colours. In the Paget process the glass is coated with points of colour laid in a regular pattern. This is called the screen plate and is placed in the dark slide face to face with a colour-sensitive plate, and the expression is made through the screen plate. From the negative so obtained a monochrome positive plate is made, and this is bound up in contact with one of the screen plates. Some little experience is necessary to obtain correct register. Each process has its difficulties. Most photographers consider that the Autochrome plate gives the nearest approach to natural colours, but the slides are very dense and require an intense light to show them well. The Paget plate is much more transparent and possesses the advantage that any number of positives may be made from the negative, whereas the Autochrome plate having been converted into a positive cannot be multiplied, resembling in that particular the earliest form of photography, the Daguerrotype. None of these colour processes are suitable for photographing objects in motion. The exposures may be reckoned as from fifteen to twenty times as long as with an ordinary slow plate. III. GEOLOGY. _By THE LATE W. T. BLANDFORD, F.R.S._ _Revised by PROF. E. J. GARWOOD, F.R.S._