DAY FLY. 
means of micrometer threads or wires extended transversely to 
each other across the field of view, as shown in fig. 4. By this 
means, the field of view was, as it were, mapped out in squares, 
like lines of latitude and longitude, upon which the magnified 
images of the objects to be delineated were seen projected. The 
draftsman having previously prepared on paper a corresponding 
system of lines, transversely intersecting each other at distances, 
one from another, determined by the scale of the intended drawing, 
he proceeded to trace the outlines of the objects, guided by the 
correspondence between the system of squares upon his paper, and 
the system of squares seen in the microscope. The outlines being 
then obtained, which could always be most conveniently done with 
a low magnifying power, which would include at once within the 
field the entire object, or objects, to be drawn, the minute details 
of form and structure, were filled up within the outlines by 
viewing the parts of the object successively with much higher 
powers. 
Neither this method, nor any other, depending on mere mecna- 
nical experience, would admit of being applied to the delineation 
of living objects, which are liable constantly to shift their positions 
and change their attitudes. To delineate these, the microscopist 
must also be an artist, and one of rather a high order; happily, 
the combination of the two qualities was not unfrequently found, 
and many beautiful representations, on a magnified scale, of the 
minuter members of the creation, have been supplied by the 
researches and talents of microscopic observers. 
35. We shall select from these one or two admirable examples 
supplied by the late Dr. Goring; and it will not be unacceptable 
to the reader, if we accompany them with a brief account of the 
objects they represent. 
36. For those who have not devoted attention to the history of 
the insect world, it may be well here to premise, that these little 
creatures are generally produced from eggs, and that, unlike all 
other members of the animal kingdom, they pass during their life 
through three stages of existence, in which their forms, habits, 
nourishment, and dwellings, differ one from another, for the same 
individual insect, as widely as do those of a crocodile and a 
peacock. 
37. There is a certain little insect of the class of flies, called a 
day-fly, because the duration of its life, from the moment it 
attains the third and perfect stage of its existence never exceeds 
a day. 
This insect deposits its eggs in water, well knowing, as it would 
seem, that its young, when hatched, are destined to be aquatic 
animals, although it is itself one of the gayest animals of the air. 
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