76 THE A B C OF PHOTO - MICROGRAPHY 



piece. Radiant, flat side of coal oil flame ; no 

 condenser of any kind; a rapid landscape 

 plate, without color screen. Exposure, eight 

 minutes; developer, pyrogalol; amplification, 

 60 diameters. 



My friend, Dr. A. C. Mercer, of Syracuse, 

 N. Y., has suggested a method of making 

 prints giving the effects of direct dark field illu- 

 mination from any negative by ordinary trans- 

 mitted light, if the subject be suited to this 

 treatment. Details which are brought out by 

 the oblique lighting of the paraboloid cannot, 

 of course, be shown by this method, but it is 

 most useful in very many instances and unlim- 

 ited as to the amount of permissible amplifi- 

 cation. Fig. 14 is a beautiful example of this 

 method of obtaining dark field effects. The 

 specimen, one of the beautiful discoid diat- 

 oms, Heliopelta, was first photographed by 

 transmitted light under an amplification of 

 225 diameters. From the negative thus made 

 a positive was printed by contact on a glass 

 plate of slow emulsion, from which in turn 

 the paper print shown in the reproduction 

 was made. It speaks well for the process, 

 which is particularly valuable to those who 

 have no paraboloid for direct work, as is the 

 usual case with the more modern micro- 



