LINE PHOTOGRAPHS 103 



up the picture into parallel lines which vary in width 

 according to the density of the photograph at each 

 point. Thus, in the case of a portrait, the dark 

 coat would be represented by wide lines close 

 together, a light part of the face by fine lines 

 correspondingly wide apart, the distance between 

 the centre of any two lines being always constant. 

 If there are fifty lines per inch, no line can be wider 

 than 5\j inch ; as a matter of fact, it should not 

 be wider than -^ t, where t is the width of the point 

 of the stylus, otherwise a dark' part of the picture 

 would appear in the telegraph as a " dead black." 

 The print from the half-tone negative is now pre- 

 pared by printing from it (in arc light) upon a 

 sheet of thick lead foil coated with a thin layer of 

 fish-glue rendered sensitive to light by means of 

 a soluble bichromate. When printed it is held 

 under a tap, and all the unexposed parts dissolve 

 away, i.e., the parts in between the lines. The 

 print is now dried and placed between two polished 

 steel plates, and put into a press. This causes the 

 glue image to sink into the soft metal without dis- 

 torting it, and a smooth commutator surface is 

 obtained, which therefore offers no resistance to 

 the stylus, while it allows of very intimate con- 

 tact between the stylus and the picture. 



The image is made negative for the telec- 

 trograph, as wherever there is bare metal a black 

 mark is produced at the receiving instrument. The 



