Appendix I 161 



(a) because the plate is insufficiently developed, () because of the 

 blackened film above the picture. Under exposure is detected by 

 the impossibility of securing a crisp picture though development 

 is pushed to excess. Development should be complete in about 

 two or three minutes, but an under-exposed plate after this time 

 would still fail to show a good sharp picture, and further develop- 

 ment would simply result in the "fogging" of the plate by a 

 blackening similar to that mentioned above. 



Both faults, if small, would therefore produce much the same 

 type of plate, though experience enables one to appreciate small 

 differences which indicate the kind of error ; but no trouble should 

 be experienced by a beginner in deciding whether his plate is 

 correctly exposed or not while he is developing it. 



Many methods exist of correcting errors of exposure to some 

 extent, while the development is in progress, but the best advice 

 that can be given to a maker of lantern slides is to determine by 

 a few experiments the exposure required for a good negative, and 

 not to vary this when found. The illumination of the object by 

 artificial light is greatly to be preferred, as it is constant, and the 

 distance of the camera from the object makes little difference in 

 this case. With stop//22, a 3" length of magnesium ribbon burnt 

 at a distance of a foot from the illustration, on each side of the lens 

 (which must, of course, be protected from the direct rays of the 

 burning ribbon) has been found to give the best results, natural 

 light being totally screened off. An unsatisfactory negative, i.e. 

 one where the picture shows, but the ground is not sufficiently 

 dense to stop out the light when printing, may frequently be 

 improved by the process of intensification, and as our work is not 

 concerned with the artistic side of photography, but merely with 

 the obtaining of an absolutely black and white copy of a black and 

 white object, intensification cannot well be over-done. 



The process is carried out as follows : After washing the 

 negative well in running water, it is laid film upwards in a 

 dish and a saturated solution of mercuric chloride poured over it. 

 Care should be taken to avoid air-bubbles, and these are frequently 

 so small as to be overlooked, consequently it may be well to brush 

 the film lightly with the tip of the third finger while under the 

 mercury solution, care being taken not to press heavily. The 

 solution should contain no suspended matter, and as it may be 

 used again for a similar operation, it should be returned to the 

 stock bottle through a. filter. 



The film will become grey and then white, when an apparent 

 reversal of the image will have taken place ; that is, black lines on 



M 



