ON SELF-RECORDING MAGNETOGRAPHS. 217 



begin to appear within five minutes of its being laid on the solution, and 

 will be finished in a few hours. It may, however, sometimes be requisite, 

 if the light has been feeble, to prolong the development for a day or more. 

 If the dish be perfectly clean, the developing solution will remain active for 

 the whole of this time, and when used only for a few hours, will be quite 

 clear and colourless, or with the faintest tinge of brown ; a darker appear- 

 ance indicates the presence of dirt. The progress of the development may 

 be watched, by gently raising one corner with the platinum spatula, and 

 lifting the sheet up by the fingers. This should not be done too often, as 

 there is always a risk of producing stains on the surface of the picture. I 

 prefer allowing the development to go on until the black is rather more in- 

 tense than ultimately required, as it is generally toned down in the fixing bath. 



51. As soon as the picture is judged to be sufficiently intense, it must be 

 removed from the gallo-nitrate, and laid on a dish of water (not necessarily 

 distilled). In this state it may remain until the final operation of fixing, 

 which need not be performed immediately, if inconvenient. After being 

 washed once or twice, and dried between clean blotting-paper, the picture 

 will remain unharmed for weeks, if kept in a dark place. 



52. Thejixingr bath is composed of a saturated solution of hyposulphite of 

 soda diluted with its own bulk of water. Into this the sheets are to be com- 

 pletely immersed, until the whole of the yellow iodide of silver has been 

 dissolved out. This operation need not be performed by yellow light; day- 

 light is much better for showing whether the picture be entirely fixed. This 

 will take from a quarter of an hour to two hours, according to the time the 

 bath has been in use. 



It will be well not to put too many sheets into the bath at once, in order 

 to avoid the necessity of turning them over to allow the liquid to penetrate 

 every part. 



When fixed, the sheet, if held up between the light and the eye, will 

 present a pure transparent appearance in the white parts. 



The fixing bath gradually becomes less and less active by use, and then 

 its action is very energetic on the dark parts of the picture, attacking and 

 dissolving them equally with the unchanged iodide. When this is the case 

 it should be put on one side (not thrown away), and a fresh bath made. 



53. After removal from the fixing bath, the sheets must be well-washed. 

 In this operation, the effect depends more upon the quantity of water used 

 than upon the duration of the immersion. When practicable, it is a good 

 plan to allow water from a tap to flow over the sheets for a minute or two, 

 and having thus got rid of the hyposulphite of soda from the surface, to 

 allow them to soak for about ten minutes in a large dish of hot water. 



54. They are then to be dried by hanging up by a crooked pin, as after 

 iodizing. W T hen dry, they will present a very rough and granular appearance 

 in the transparent parts ; this is removed by melting the wax, either before 

 a fire, or, what is far better, by placing them between blotting-paper, and 

 passing a warm iron over them ; by this means the white parts will recover 

 their original transparency. 



55. The picture, arrived at this stage, may be considered finished, as far 

 as is requisite for the purposes of measurement and registration ; sometimes, 

 however, it may be necessary to multiply copies, for the purpose of trans- 

 mitting to other Meteorological Observatories facsimiles of the records, or 

 at least of those containing any remarkable phenomena. I will therefore 

 now detail the method of printing photographic positives from these nega- 

 tives, premising that the process does not differ materially from that usually 

 adopted. 



