A Manual Of Photography

154 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. Mr. Jordan has also published an account of his very inge- nious plan of applying the same kind of paper to the magne- tometer or diurnal variation needle,' and several other philo- sophical instruments ; but as these applications were not at the time entirely successful, owing principally to the difficulty of finding a suitable situation for so delicate an instrument, it is thought unnecessary to occupy these pages with any particular description of the arrangements adopted, which, however, were in all essential points similar to those employed by Mr. Ronalds, and adopted in some of our magnetic and meteorological observatories. Those of Mr. Brooks are of somewhat more refined a character, and require special notice. A reflector is attached to the end of a delicately suspender magnet : this reflects a pencil of strong artificial light upon photographic paper placed between two cylinders of glass, which are kept in motion by a small clock arrangement. As the paper moves in a vertical direction whilst the magnet oscillates in a horizontal one, a zigzag line is marked on the paper; the extent of movement on either side of a fixed line showing the deviation of the magnet for every hour of the day. By means of this arrangement many most remarkable phenomena connected with terrestrial magnetism have been discovered, and since the methods of adjustment have been rendered more perfect, and the inven- tion applied to a great variety of instruments, we may hope for vet more important results. . ' „... .. The registration of the ever-varying intensity of the light is so important a subject, that it has occupied the attention of several eminent scientific observers. Sir John Herschel and Dr Daubeny have applied their talents to the inquiry, and devised instruments of much ingenuity for the purpose. The instrument constructed by Sir John Herschel, which he has named an actinograph, not only registers the direct effect of solar chemical radiation, but also the amount of general actinic power in the visible hemisphere; one portion of the apparatus being so arranged that a sheet of sensitive paper is slowly moved in such a direction, that the direct rays of the sun, when unobscured, may fall upon it through a small slit made in an outer cylinder or case, while the other is screened from the incident beam. The paper being fixed on a disc of brass, made to revolve by watch-work, is affected only by the light which “ emanates from that definite circumpolar region of the sky to which it may be considered desirable to limit the observation, and which is admitted, as in the other case, through a fine slit in the cover of the instrument. 1 See the Sixth Annual Report of the Royal Cornwall Polytechnic Society.