PRACTICAL PHOTO-MICROGRAPH Y. 141 



The front of the lantern must be so arranged that the lens, 

 used for projection can be racked to at least twice its own 

 focal length from the original. 



In this process of enlarging (by the optical lantern) the rela- 

 tion between focal length of the projection lens and area of 

 the original plays no part; a diaphragm is of little optical 

 advantage, and to proper selection and use of our condenser 

 we must look for success. We are as much incompetent here 

 as in daylight enlargement to give rules for exposure, but the 

 one rule we did give holds equally good here. 



If by either daylight or lantern process we wish to produce 

 an enlarged negative, two courses are open to us. 1st : We 

 may make from our original (presumably a negative) an en- 

 larged positive, and from that we may print " by contact " a, 

 negative. For this process we recommend a slow gelatine- 

 bromide plate, such an emulsion as is made for lantern-slides 

 answering admirably ; or we may produce our large contact- 

 negative by the carbon process, or on gelatine chloride emul- 

 sion. (For the carbon process consult "Processes of Pure 

 Photography," or u Burton's Guide to Practical Printing," etc.) 

 This is probably the better course if we enlarge by daylight. 

 2d : We may in the first place make a small positive by contact 

 or reduction (according to the size of our original negative), 

 and from this small positive we may make an enlarged nega- 

 tive on any suitable sensitive plate or film. A small positive 

 such as would be a good lantern-slide is not so well adapted 

 for this purpose as one almost fogged ; that is to say, the posi- 

 tive for this purpose should be very fully exposed, more heavily 

 developed than a lantern -slide, full of detail without absolutely 

 clear glass; and presenting no violent contrasts. 



Whatever be the sensitive material used to receive the en- 

 largement, the after operations are the same as those given by 

 us under the heads of development, etc., of the various sensi- 

 tive materials treated in other chapters. Bromide paper and 

 Transf erotype, for instance, are treated just as if they had been 

 exposed by contact, errors in exposure will manifest them- 

 selves in the same way in both cases. 



If very sensitive emulsion is used, as for example an ordin- 



