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 mecha- 

 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. 



75 



