Sir Benjamin son's method. surprised the public. They pass through many "opaque" substances, and are stopped by many "transparent" bodies. They seem incapable of reflection or refraction, but they can be deflected by a magnet placed in their path. And here comes the main difference between the new "X"-rays and the older cathode rays, for the former are not deflected by a magnet. The cathode rays, too, seem to have much less penetrative power in air, for the "X"-rays produce results at a distance from their source where the cathode rays have ceased to be active. For photographing the bones within the flesh it is not necessary to use invisible light, and it is probable that the method of Sir BenRichard jamin Richardson, described before the British Association in 1868, may be modified to give much better results than will ever be obtained with "X"-rays. In this case also the diseased structure can be seen and need not be photographed. By placing the body in an aperture with an intense light behind it, and the observer in an otherwise darkened room, it is possible to see fractures of bones, etc. PHOTOGRAPHY ALFRED RUSSEL WALLACE T Spectrum Analysis. HE improvements in the mode of production of light for common use are sufficiently new and remarkable to dis- Photogtinguish this century from all the ages that raphy and preceded it, but they sink into insignificance when compared with the discoveries which have been made as to the nature of light itself, its effects on various kinds of matter leading to the art of Photography, and the complex nature of the Solar Spectrum leading to Spectrum Analysis. This group of investigations alone is sufficient to distinguish the present century as an epoch of the most marvellous scientific discovery. theory of Although Huygens put forward the wave- The wavetheory of light more than two hundred years light. ago, it was not accepted, or seriously studied, till the beginning of the present century, when it was revived by Thomas Young, and was shown by himself, by Fresnel, and other mathematicians, to explain all the phenomena of refraction, double-refraction, polarization, diffraction, and interference, some of which were inexplicable on the Newtonian theory of the emission of material particles, which had previously been almost universally accepted. The complete establishment of the undulatory theory of light is a fact of the highest importance, and will take a very high place among the purely scientific discoveries of the century. From a more practical point of view, however, nothing can surpass in interest and importance the discovery and continuous improvement of the Photographic art, which has now reached such a development that there is hardly any science or any branch of intellectual study that is not indebted to it. A brief sketch of its origin and progress will therefore not be uninteresting. The fact that certain salts of silver were Rudiments darkened by exposure to sunlight was known to the alchemists in the Sixteenth Century, of the art. and this observation forms the rudiment from which the whole art has been developed. The application of this fact to the production of pictures belongs, however, wholly to our own time. In the year 1802, Wedgwood described a mode of copying paintings on glass by exposure to light, but neither he nor Sir Humphry Davy could find any means of rendering the copies permanent. This was first effected, in 1814, by M. Niepce of Châlons, The Da- but no important results were obtained till 1839, when Daguerre perfected the beautiful guerrotype. Per process known as the Daguerrotype. collodion About the same time a method was found for photographing leaves, lace, and other semi-transparent objects on paper, and ren- The dering them permanent, but this was of com- film. paratively little value. In the year 1850, the far superior collodion-film on glass was perfected, and negatives were taken in a cameraobscura, which, when placed on black velvet, or when coated with a black composition, produced pictures almost as perfect and beautiful as the daguerrotype itself, and at much less cost. Soon afterward positives were printed from the transparent negatives, on suitably prepared paper; and thus was initiated the process which, with endless modifications and improvements, is still in use. The main advance has been in the increased sensitiveness of the photographic plates, so that, first, moving crowds, then breaking waves, running horses, and other quickly moving objects were taken, while now a bullet fired from a rifle can be photographed in the air. With such marvellous powers, photography Scientific has come to the aid of the arts and sciences in uses of pho- Ways which would have been perfectly incontography. ceivable to our most learned men of a century Aids in astronomy. ago. It furnishes the Meteorologist, the Physicist, and the Biologist with self-registering instruments of extreme delicacy, and enables them to preserve accurate records of the most fleeting natural phenomena. By means of successive photographs at short intervals of time, we are able to study the motions of the wings of birds, and thus learn something of the mechanism of flight; while even the instantaneous lightning-flash can be depicted, and we thus learn, for the first time, the exact nature of its path. Perhaps the most marvellous of all its achievements is in the field of astronomy. Every increase in the size and power of the telescope has revealed to us ever more and more stars in every part of the heavens; but, by the aid of photography, stars are shown which no telescope that has been, or that probably ever will be, constructed, can render visible to the human eye. For by exposing the photographic plate in the focus of the object glass for some hours, almost infinitely faint stars impress their image, and the modern photographic star-maps show us a surface densely packed with white points that seem almost as countless as the sands of the seashore. Yet every one of these points represents a star in its true relative position to the visible stars |