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TEXT-BOOK OF ENTOMOLOGY 



sion ; the plane is therefore at right angles to the axis of the object-glass. The 

 breadth of the wing can be seen in its entirety ; the nervures can be counted, 

 and the rounding off of the extremities of the wings is perfectly obvious. On 

 the other hand, the flying insect moves its wings in a horizontal direction, and 

 owing to the resistance of the air the expanse of the wings is obliquely disposed, 



and only the projection of its surface can be seen in the photograph. This is 

 why the extremity of the wings appears as if it were pointed, while the other 

 parts look much narrower than normal. The extent of the obliquity can be 

 measured from the apparent alteration in width, for the projection of this plane 

 with the vertical is the sine of the angle. From this it may be gathered that 

 the right wing (Fig. 108, third image) was inclined at an angle of about 50 

 with the vertical, say 40 with the horizontal. This inclination necessarily 

 varies at different points of the trajectory and must augment with the rapidity 



