06 J. Waterliouse — The Application of Photography [No. 2, 



the plate ; the tympan is lowered and the impression pulled in the ordinary 

 way. The plate is then damped and inked in again, and so on. 



Half-tone subjects are treated in the same manner, but it is sometimes 

 advisable to use two kinds of ink of different consistence or depth of colour ; 

 a stifE or dark ink gives force to the shadows, while a thin or lighter 

 coloured one will bring out the delicate half-tones. KoUers made of gelatine, 

 glycerine and castor-oil may be used wdth advantage, as they drive the ink 

 better into the hollows of the lines than the leather rollers. Capt. Abney, 

 who has given great attention to these processes, says that the great 

 secret of producing good results is to have the command of first rate rollers. 

 Glazed enamelled paper is generally used for printing half-tone subjects, 

 but in some cases unenamelled paper answers well. The most suitable 

 paper for printing seems to depend partly on the composition of the sensi- 

 tive surface and partly on the ink. 



One of the great drawbacks to the extended use of the photocollotype 

 process for the reproduction of maps is the difficulty of making corrections 

 on the plates. When the printing surface is a metal plate or lithographic 

 stone, upon w^hich a map has been either engraved, zincographed or litho- 

 graphed, additions and erasures may easily be made without any risk of the 

 loss of the printing surface or even of much damage to it. With the tender 

 gelatine films the case is different, and although writing or simple lines 

 may be inserted without much difficulty, it would be almost impossible to 

 successfully alter gradation of shade or to insert shaded details. On the 

 other hand, the taking out of details must be done by some chemical means 

 which must always be attended with the imminent risk of raising the gelatine 

 film from its support and the consequent utter destruction of the printing 

 plate. 



As maps, almost more than any other printed subject, require that it 

 shall always be possible to make corrections on the printing plates, it is 

 obvious that the use of any process which will not permit of this being 

 done must be confined more to the reproduction of maps already printed or 

 of an ephemeral character than to the preparation of new or standard 

 ones. And thus, though photocollotype is admirably adapted for repro- 

 ducing copies of old or other special maps, which are, or can be, finished 

 once and for all, it is not suited for maps on which corrections are likely to 

 be required. 



With the plates prepared as described we have found that details may 

 be inserted by two or three methods. The first is by writing in the re- 

 quired additions on the dry gelatine surface, using an ink composed of 

 bichromate of potash, either alone or coloured with Indian ink. After the 

 insertion of the additions the plate is exposed to the light for a few minutes 

 to reduce the bichromate, and may then be washed and printed as usual. 



