28 On a Photographic Process. 



Art. VI. — Notes on a Simplification of a Photographic 



Process used with Self-registering Instruments. 



By R. L. J. Elleey, Esq. 



[Bead 14th March, 1870.] 



Since the adoption of photography as a means for obtain- 

 ing continuous and automatic records of magnetic meteoro- 

 logical and other phenomena, the observatory work in those 

 branches of physical science has undergone almost a com- 

 plete revolution. A fair knowledge of the theory and 

 practice of photography has now become essential to the 

 observer, and no public observatory of any pretensions 

 can be considered complete without its photographic room. 



The photographic method of registration was first adopted 

 in our observatory in August, 1867, in connection with an 

 instrument (which I have already described) for measuring the 

 force and variations of atmospheric electricity, and subse- 

 quently for the magnetic instruments, the self-registering 

 barometer, and lately also for wet and dry bulb thermo- 

 meters. At the present time about 20 sheets (6 inches by 

 13 inches) are prepared, developed, and finally treated every 

 week. Artificial light, either from gas, oil, or kerosene, is- 

 always used in this kind of photography — in our observatory 

 the former is used. 



The instruments are so arranged that the light, from a 

 peculiar kind of burner, falls on to mirrors atiixed to the 

 movable and sensitive parts of the apparatus, or passes 

 through transparent spaces which move with the indicators 

 of the particular instrument, after which it is focussed or 

 condensed, so as to fall on to the photographic paper in the 

 form of an intense dot or line of light. In the Electrographs 

 and Magnetographs, the light first passes through a narrow 

 slit and an achromatic lens, then falls on to the mirror, which 

 reflects it as a line of light towards a cylinder, around which 

 the sensitive paper is fixed ; it is intercepted, however, by 

 a cylindrical lens, which converges the line of light to a dot 

 on the paper. As the mirror moves with the magnets or 

 electrograph pendulum the dot will fall on different parts of 

 the cylinder, which is caused to revolve once in 24 or 48 

 hours by clockwork^ — a curve or crooked line is therefore 

 traced on the paper, showing the deviation of the magnets, 

 &c., in the 24 or 48 hours. In the Barograph the light 

 passing through the vacuum above the Mercury Column, is 

 converged to a sharp line on the cylinder, and is elongated or 

 shortened as the mercury rises or falls ; in the Thermographs 



