ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
873 
the objects while the sensitive plate in the shutter is ready to receive 
the photographic impression. The details of the arrangement have 
already been described in this Journal, 1886, p. 842. 
In front of the Microscope is a stand on which are mounted a mirror, 
a condensing lens, and an iris diaphragm, which serve for the proper 
regulation of the light. 
Microscopical Illustrations.* — Mr. H. L. Tolman writes : — “ One 
of the most difficult features in connection with the illustration of 
scientific articles is the reproduction of the photomicrographs or camera- 
lucida drawings. If the object contains a large amount of detail, a 
photograph will be the only way by which all the minutise can be 
preserved, but no woodcut can entirely reproduce the original. Of 
course, it is not always necessary that everything which is seen under 
the Microscope should be seen in the book illustration, and just here 
is the point where authorities differ on the requisites of a good wood- 
cut or engraving. Some hold that only the salient parts of an object 
need be represented ; in fact, that the picture is better for having 
omitted from it all but the few leading features, to which the writer 
desires to call attention. Others claim that the picture should repre- 
sent just what the eye sees under the Microscope, free from any of the 
possible or intentional errors of the artist. There is, undoubtedly, 
much to be said on both sides of the question. 
Those who have studied the astonishing cuts called “ diagrammatic 
representations,” the counterpart of which they vainly search for in 
nature, will be strongly in favour of any method which reproduces an 
object so that it can be recognized, and the tendency of the art of the 
present day seems to be in this direction. Fortunately, with this 
demand comes an improvement in the manner of reproducing photo- 
graphs and drawings of every kind, which deserves a somewhat ex- 
tended notice. This is by what is known as the half-tone process, 
which consists of a photographic copy of the original on a zinc or 
copper plate and then etching the plate until the drawing appears in 
relief, and is printed from like an electrotype. In order to convert the 
smooth, 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 subjects a coarse screen is appropriate, while for those 
with great detail and delicate graduations of shades a fine screen is 
required. 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 1/100 to 1/150 in. Obviously, for nearly all micro- 
scopical 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 
1892. 
Amor. Mon. Micr. Journ., xiii. (1892) pp. 155-7. 
6 N 
