156 THE AMERICAN MONTHLY [July, 



soft shades of a photograph into a form which will prevent 

 them from printing a solid black, they are broken up into a 

 series of dots more or less close, by the interposition of a 

 finely ruled screen in the camera just in front of the dry plate. 

 On the character of this screen depends in great measure the 

 quality of the finished picture, and the skill of the process worker 

 is best shown by a proper selection of the screen to illustrate the 

 landscape or portrait which is to be reproduced. For bold sub- 

 jects a coarse screen is appropriate, while for those with great 

 detail and delicate graduations of shades a fine screen is re- 

 quired. These screens are either ruled on paper and copied by 

 the wet-plate process on glass, or directly on glass, the lines 

 varying in distance from i-ioothto i-i5othof an inch. Obviously 

 for nearly all microscopical photographs the fine screens must 

 be used, but it will be found that some of the contrast must be 

 sacrificed thereby. 



Of course, there are some subjects which cannot be reproduced 

 by the half-tone process. Where the oljject is entirely novel 

 and the peculiar grain-like structure of the picture might cause 

 erroneous conclusions, where there is a large amount of fine detail, 

 or where, as in diatoms, the structure consists of a series of fine 

 lines, or minute dots, elevations or apertures, then this process is 

 not so applicable. But even here it is serviceable if only the 

 general appearance of the object is desired. One suggestion may 

 be allowed to be otlered — that in studying all pi"ocess work the 

 best effect is given if the picture is held at a rather greater dis- 

 tance from the eyes than the distance of normal vision, /. e., ten 

 inches. By this means the attention of the observer will not be 

 distracted by seeing the individual dots or points of which the 

 image is made up, but he will still be enabled to appreciate all 

 the delicate effects of light and shade. 



There are several methods in common use for producing what 

 are called half-tone prints. The bitumen process, though com- 

 mercially but little vised on account of its slowness, is one of the 

 best for reproducing detail, especially since Valenta has discovered 

 some valuable improvements, while the numerous so-called 

 enamel processes which have recently been introduced, modifica- 

 tions of the gelatine process, are valuable in that they are rapid 

 and cheap. The cheapness, of course, is one of the strongest 

 recommendations for the use of any half-tone process, as a cut 

 which on wood might cost from $io to $20 could be put into 

 a half-tone for $3 to $5. Another advantage is that the amount 

 of detail in a picture vv'ould make no difference in the price of a 

 zinc plate, it being sold for a certain price per square inch, while 

 the cost of a wood-cut would depend very largely on the ffneness 

 of the detail which was to be reproduced. This kind of illustra- 

 tion has already been largely used in medical work, and the A. 

 M. M. J. a year or two ago republished some half-tone cuts by 

 the author of photo-micrographs of sections of woods, showing 



