ON SELF-RECORDING MAGNETOGRAPHS. 219 



62. When a positive is to be printed from a negative, let the glass of 

 the pressure frame be perfectly cleaned and freed from dust on both sides, 

 then lay the negative on it, with its back to the glass. On it place a sheet 

 of positive paper, with its sensitive side down. Then, having placed over, 

 as a pad, several sheets of blotting-paper, screw the back down with 

 sufficient force to press the two sheets into close contact, but of course not 

 so as to endanger the glass. Now place the frame in the sun, so that the 

 light can fall perpendicularly on the glass, and allow it to remain there 

 until it is judged to have been exposed long enough. 



63. No rule can be laid down for the proper time of exposure ; it will 

 depend upon the quality of the light and intensity of the negative ; some 

 pictures being completed in a few minutes, others requiring upwards of 

 half an hour. The printing should always go on until the picture is 

 several shades darker than ultimately required. A very little experience 

 will enable the operator to judge so well of the quality of the light, as 

 hardly ever to have a failure. If the two sheets of paper be stuck together 

 in two or three places at the edges with small pieces of gummed paper, the 

 frame can be removed to the dark room, and the progress of the sheets 

 examined ; but this is always attended with some danger, for unless th«y 

 are replaced without having been shifted one from the other, there will be a 

 double image. 



64. As soon as the picture is considered to be printed sufficiently deep, 

 it has to be fixed. 



The fixing bath consists of 



Saturated solution of hyposulphite of soda . . . . 10 ounces. 

 Water , 30 ounces. 



This bath will be found to fix the pictures perfectly, but they will 

 generally be of a reddish tint; if it be thought desirable to obtain the 

 pictures of some shade of dark brown or black, it will be necessary to 

 employ a bath made as follows : — 



Saturated solution of hyposulphite of soda .... 10 ounces. 



Water 10 ounces. 



Exhausted positive exciting solution (61) .... 10 ounces. 



Mix these together, and then add the following : — 



Water 10 ounces. 



Chloride of gold 20 grains. 



taking care in mixing to pour the solution of gold into the solution of 

 hyposulphite, and not the latter into the former, or another decomposition 

 will be produced. 



Pour this mixture into a dish, and lay the positive carefully on it, face 

 downwards. As soon as it is thoroughly damp (which may be known by 

 its becoming perfectly flat after having curled up), immerse it totally in the 

 liquid. 



65. The pictures should not be too crowded in the bath, as they are very 

 apt to become irregularly coloured in places where the hyposulphite has not 

 had free access during the whole of the time. When first put in, the colour 

 will change to a light brown, and in the course of some time, varying from 

 ten minutes to two or three hours, it will pass through the different shades 

 of brown to black and purple, gradually fading in intensity during the time. 

 It will be necessary to allow the picture to remain in this bath for ten minutes 

 at least, in order that it may be perfectly fixed. After this time, its stay 



